Telecommunications systems and methods

ABSTRACT

A wireless telecommunications system including a base station and a terminal device and employing a radio interface having a downlink radio frame structure including radio subframes including an arrangement of reference symbols for channel estimation. The base station is configured to determine a period of time for which certain terminal device specific data are not scheduled for transmission to the terminal device and to communicate this information to the terminal device through selective suppression of at least one reference symbol. Different reference symbol(s) may be suppressed to indicate different periods of time. The terminal device is configured to monitor the reference symbols transmitted by a base station to identify where reference symbols are suppressed, to determine a period of time for which the terminal device is not expected to receive certain types of data and enter a reduced activity mode for that period to conserve processing and power resources.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to British Patent Application1216937.1, filed in the UK IPO on 21 Sep. 2012, the entire contents ofwhich being incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to methods, systems and apparatus forconveying information from a base station to a terminal device in awireless telecommunications system to control a reduced activity mode atthe terminal device to conserve resources.

Third and fourth generation mobile telecommunication systems, such asthose based on the 3GPP defined UMTS and Long Term Evolution (LTE)architecture are able to support more sophisticated services than simplevoice and messaging services offered by previous generations of mobiletelecommunication systems.

For example, with the improved radio interface and enhanced data ratesprovided by LTE systems, a user is able to enjoy high data rateapplications such as mobile video streaming and mobile videoconferencing that would previously only have been available via a fixedline data connection. The demand to deploy third and fourth generationnetworks is therefore strong and the coverage area of these networks,i.e. geographic locations where access to the networks is possible, isexpected to increase rapidly.

The anticipated widespread deployment of third and fourth generationnetworks has led to the parallel development of a class of devices andapplications which, rather than taking advantage of the high data ratesavailable, instead take advantage of the robust radio interface andincreasing ubiquity of the coverage area. Examples include so-calledmachine type communication (MTC) applications, which are typified bysemi-autonomous or autonomous wireless communication devices (i.e. MTCdevices) communicating small amounts of data on a relatively infrequentbasis. Examples include so-called smart meters which, for example, arelocated in a customer's house and periodically transmit information backto a central MTC server data relating to the customers consumption of autility such as gas, water, electricity and so on. Further informationon characteristics of MTC-type devices can be found, for example, in thecorresponding standards, such as ETSI TS 122 368 V10.530 (2011-07)/3GPPTS 22.368 version 10.5.0 Release 10) [1]. Some typical characteristicsof MTC type terminal devices/MTC type data might include, for example,characteristics such as low mobility, high delay tolerance, small datatransmissions, infrequent transmission and group-based features,policing and addressing.

Whilst it can be convenient for a terminal such as an MTC type terminalto take advantage of the wide coverage area provided by a third orfourth generation mobile telecommunication network there are at presentdisadvantages. Unlike a conventional third or fourth generation terminaldevice such as a smartphone, an MTC-type terminal is preferablyrelatively simple and inexpensive and able to operate on relatively lowresources (e.g. low power consumption). The type of functions performedby the MTC-type terminal (e.g. collecting and reporting back data) donot require particularly complex processing to perform, and furthermoreare typically not time-critical. However, third and fourth generationmobile telecommunication networks typically employ advanced datamodulation techniques on the radio interface which can be power hungryand require more complex and expensive radio transceivers to implement.It is usually justified to include such complex transceivers in asmartphone as a smartphone will typically require a powerful processorto perform typical smartphone type functions. However, as indicatedabove, there is now a desire to use relatively inexpensive and lesscomplex devices able to operate with low resource usage to communicateusing LTE type networks.

Known techniques for lowering power consumption in LTE-type terminaldevices include the discontinuous reception (DRX) mode and themicrosleep mode. The DRX mode involves controlling a terminal device toenter an idle mode through Radio Resource Control (RRC) signalling.Drawbacks of the DRX mode include reconnection latency as the terminaldevice moves from idle mode back to connected mode as well as initiationdelays and signalling overhead associated with the RRC signalling. Thesecan mean DRX is an efficient mechanism for saving terminal deviceresources when the device is to be idle for relatively long periods, forexample, for hundreds of milliseconds or longer, but the DRX mode isless efficient for controlling shorter duration periods of reducedterminal device activity. The microsleep mode involves a terminal devicedetermining from a control region of a subframe that there is nouser-plane data for the terminal device in the remainder of thesubframe, and suspending decoding of the remainder of the subframeaccordingly. The microsleep mode is thus applicable for timescales whichare much shorter than the DRX mode (i.e. microsleep can be applied on aper subframe basis). Furthermore, there is no RRC signalling overheadassociated with microsleep. However, the microsleep mode requires aterminal device to decode a control region of each subframe to determinewhether to microsleep for the remainder of the subframe and thisrestricts the extent to which the terminal device can save power. Forexample, unlike the DRX mode, the microsleep mode cannot be used toconfigure a terminal device into a continuous reduced-activity state fora number of subframes.

In view of the above-identified drawbacks of existing schemes, there istherefore a need for alternative approaches for controlling a terminaldevice communicating with a base station to enter a reduced activitymode.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a methodof operating a base station to convey to a terminal device informationregarding a period of time for which a type of terminal device specificdata is not scheduled for transmission to the terminal device in awireless telecommunications system employing a radio interface includingan arrangement of downlink reference symbols, the method comprising:determining a period of time for which the type of terminal devicespecific data are not scheduled for transmission to the terminal device;selecting at least one reference symbol from the arrangement of downlinkreference symbols in dependence on the determined period of time; andsuppressing transmission of the at least one reference symbol toindicate to the terminal device the period of time for which the type ofterminal device specific data are not scheduled for transmission to theterminal device.

In accordance with some embodiments the selection of the at least onereference symbol for which transmission is to be suppressed is alsobased on an association between different ones of the reference symbolsand different potential periods of time for which the type of terminaldevice specific data are not scheduled for transmission to a terminaldevice.

In accordance with some embodiments the association between differentones of the reference symbols and different potential periods of timefor which the type of terminal device specific data are not scheduledfor transmission to a terminal device is pre-defined for the wirelesstelecommunications system.

In accordance with some embodiments the association between differentones of the reference symbols and different potential periods of timefor which the type of terminal device specific data are not scheduledfor transmission to a terminal device is established by the base stationand communicated to the terminal device in prior signalling.

In accordance with some embodiments the association between differentones of the reference symbols and different potential periods of timefor which the type of terminal device specific data are not scheduledfor transmission to a terminal device is defined in a look-up table.

In accordance with some embodiments the at least one reference symbolfor which transmission is to be suppressed comprises more than onereference symbol selected according to a mapping between differentcombinations of reference symbols and a plurality of potential periodsof time for which the type of terminal device specific data are notscheduled for transmission to a terminal device.

In accordance with some embodiments the base station comprises multipleantenna ports for transmitting the reference symbols and the antennaports for which transmission of the reference symbols is to besuppressed is selected in dependence on the determined period of timefor which the type of terminal device specific data are not scheduledfor transmission to a terminal device and/or an identity for theterminal device.

In accordance with some embodiments the reference symbols comprisecell-specific reference symbols and/or terminal device specificreference symbols and/or demodulation reference symbols and/or channelstate information reference symbols and/or positioning referencesymbols.

In accordance with some embodiments the selection of the at least onereference symbol for which transmission is to be suppressed is alsobased on a start time for the period of time for which the type ofterminal device specific data are not scheduled for transmission to theterminal device relative to a time at which the transmission of the atleast one reference symbol is to be suppressed.

In accordance with some embodiments the selection of the at least onereference symbol for which transmission is to be suppressed is alsobased on a further period of time for which the type of terminal devicespecific data are not scheduled for transmission to a terminal devicesuch that the suppressed transmission of the selected at least onereference symbol indicates to the terminal device multiple periods oftime for which the type of terminal device specific data are notscheduled for transmission to the terminal device.

In accordance with some embodiments the multiple periods of time followa pattern defined according to the at least one reference symbolselected for suppressed transmission.

In accordance with some embodiments the at least one reference symbolfor which transmission is to be suppressed is also selected based on anidentity for the terminal device for which the type of terminal devicespecific data are not scheduled for transmission during the determinedperiod of time.

In accordance with some embodiments the identity for the terminal deviceuniquely identifies the terminal device.

In accordance with some embodiments the identity for the terminal deviceidentifies a group of terminal devices of which the terminal device is amember.

In accordance with some embodiments an association between the terminaldevice and the group of terminal devices of which the terminal device isa member is established by signalling between the base station and theterminal device.

In accordance with some embodiments an association between the terminaldevice and the group of terminal devices of which the terminal device isa member is pre-defined for the wireless telecommunications system.

In accordance with some embodiments suppressing transmission of the atleast one reference symbol comprises not transmitting the at least onereference symbol or transmitting the at least one reference symbol withless power than for reference symbols which are not suppressed.

In accordance with some embodiments the radio interface has a downlinkradio frame structure comprising radio subframes.

In accordance with some embodiments a reference symbol transmitted in asubframe in which transmission of the at least one reference symbol issuppressed is transmitted with a power greater than it would otherwisebe transmitted if the at least one reference symbol had not beensuppressed.

In accordance with some embodiments the period of time corresponds witha number of subframes starting at an offset defined relative to asubframe in which a reference symbol is suppressed.

In accordance with some embodiments the at least one reference symbolfor which transmission is to be suppressed comprises at least onereference symbol in each one of more than one subframe.

In accordance with some embodiments the radio interface comprises aplurality of Orthogonal Frequency Division Multiplexing, OFDM,sub-carriers spanning a system frequency bandwidth, and wherein theradio interface supports a first carrier for communicating with a firstclass of terminal device using a first group of the OFDM sub-carriersdistributed across the system frequency bandwidth, and a second carrierfor communicating with a second class of terminal device on a secondgroup of the OFDM sub-carriers distributed across a restricted frequencybandwidth, wherein the restricted frequency bandwidth is narrower thanand within the system frequency bandwidth, and the terminal device is aterminal device of the second class operating on the second carrier.

In accordance with some embodiments the selection of the at least onereference symbol for which transmission is to be suppressed is alsobased on further information to be conveyed from the base station to theterminal device.

In accordance with some embodiments the method further comprises thebase station transmitting terminal device specific data to the terminaldevice during the period of time for which no terminal device specificdata was scheduled for transmission to the terminal device in responseto signalling received from the terminal device during this period.

In accordance with some embodiments the method further comprises thebase station receiving a channel quality indicator, CQI, from theterminal device on expiry of the period of time for which no terminaldevice specific data was transmitted to the terminal device.

In accordance with some embodiments the terminal device is amachine-type communication, MTC, terminal device.

In accordance with a second aspect of the invention there is provided abase station configured to convey to a terminal device informationregarding a period of time for which a type of terminal device specificdata is not scheduled for transmission to the terminal device in awireless telecommunications system employing a radio interface includingan arrangement of downlink reference symbols, the base stationcomprising: a scheduling unit configured to determine a period of timefor which the type of terminal device specific data are not scheduledfor transmission to the terminal device; a selecting unit configure toselect at least one reference symbol from the arrangement of downlinkreference symbols in dependence on the determined period of time; and atransmitter unit configured to suppress transmission of the at least onereference symbol to indicate to the terminal device the period of timefor which the type of terminal device specific data are not scheduledfor transmission to the terminal device.

In accordance with some embodiments the base station is configured suchthat the selection of the at least one reference symbol for whichtransmission is to be suppressed is also based on an association betweendifferent ones of the reference symbols and different potential periodsof time for which the type of terminal device specific data are notscheduled for transmission to a terminal device.

In accordance with some embodiments the association between differentones of the reference symbols and different potential periods of timefor which the type of terminal device specific data are not scheduledfor transmission to a terminal device is pre-defined for the wirelesstelecommunications system.

In accordance with some embodiments the base station is configured suchthat the association between different ones of the reference symbols anddifferent potential periods of time for which the type of terminaldevice specific data are not scheduled for transmission to a terminaldevice is established by the base station and communicated to theterminal device before suppression of the at least one reference symbol.

In accordance with some embodiments the association between differentones of the reference symbols and different potential periods of timefor which the type of terminal device specific data are not scheduledfor transmission to a terminal device is defined in a look-up table.

In accordance with some embodiments the at least one reference symbolfor which transmission is to be suppressed comprises more than onereference symbol selected according to a mapping between differentcombinations of reference symbols and a plurality of potential periodsof time for which the type of terminal device specific data are notscheduled for transmission to a terminal device.

In accordance with some embodiments the base station comprises multipleantenna ports for transmitting the reference symbols and is configuredsuch that the antenna ports for which transmission of the referencesymbols is to be suppressed is selected in dependence on the determinedperiod of time for which the type of terminal device specific data arenot scheduled for transmission to a terminal device and/or an identityfor the terminal device.

In accordance with some embodiments the reference symbols comprisecell-specific reference symbols and/or terminal device specificreference symbols and/or demodulation reference symbols and/or channelstate information reference symbols and/or positioning referencesymbols.

In accordance with some embodiments the base station is configured suchthat the selection of the at least one reference symbol for whichtransmission is to be suppressed is also based on a start time for theperiod of time for which the type of terminal device specific data arenot scheduled for transmission to the terminal device relative to a timeat which the transmission of the at least one reference symbol is to besuppressed.

In accordance with some embodiments the base station is configured suchthat the selection of the at least one reference symbol for whichtransmission is to be suppressed is also based on a further period oftime for which the type of terminal device specific data are notscheduled for transmission to a terminal device such that the suppressedtransmission of the selected at least one reference symbol indicates tothe terminal device multiple periods of time for which the type ofterminal device specific data are not scheduled for transmission to theterminal device.

In accordance with some embodiments the base station is configured suchthat the multiple periods of time follow a pattern defined according tothe at least one reference symbol selected for suppressed transmission.

In accordance with some embodiments the base station is configured suchthat the at least one reference symbol for which transmission is to besuppressed is also selected based on an identity for the terminal devicefor which the type of terminal device specific data are not scheduledfor transmission during the determined period of time.

In accordance with some embodiments the identity for the terminal deviceuniquely identifies the terminal device.

In accordance with some embodiments the identity for the terminal deviceidentifies a group of terminal devices of which the terminal device is amember.

In accordance with some embodiments the base station is configured suchthat an association between the terminal device and the group ofterminal devices of which the terminal device is a member is establishedby signalling between the base station and the terminal device.

In accordance with some embodiments an association between the terminaldevice and the group of terminal devices of which the terminal device isa member is pre-defined for the wireless telecommunications system.

In accordance with some embodiments the base station is configured suchthat suppressing transmission of the at least one reference symbolcomprises not transmitting the at least one reference symbol ortransmitting the at least one reference symbol with less power thananother reference symbol for which transmission is not suppressed.

In accordance with some embodiments the radio interface has a downlinkradio frame structure comprising radio subframes.

In accordance with some embodiments the base station is configured suchthat a reference symbol transmitted in a subframe in which transmissionof the at least one reference symbol is suppressed is transmitted with apower greater than it would otherwise be transmitted if the at least onereference symbol had not been suppressed.

In accordance with some embodiments the base station is configured suchthat the period of time corresponds with a number of subframes startingat an offset defined relative to a subframe in which a reference symbolis suppressed.

In accordance with some embodiments the at least one reference symbolfor which transmission is to be suppressed comprises at least onereference symbol in each one of more than one subframe.

In accordance with some embodiments the radio interface comprises aplurality of Orthogonal Frequency Division Multiplexing, OFDM,sub-carriers spanning a system frequency bandwidth, and wherein the basestation is configured such that the radio interface supports a firstcarrier for communicating with a first class of terminal device using afirst group of the OFDM sub-carriers distributed across the systemfrequency bandwidth, and a second carrier for communicating with asecond class of terminal device on a second group of the OFDMsub-carriers distributed across a restricted frequency bandwidth,wherein the restricted frequency bandwidth is narrower than and withinthe system frequency bandwidth, and the terminal device is a terminaldevice of the second class operating on the second carrier.

In accordance with some embodiments the base station is configured suchthat selection of the at least one reference symbol for whichtransmission is to be suppressed is also based on further information tobe conveyed from the base station to the terminal device.

In accordance with some embodiments the base station is configured totransmit terminal device specific data to the terminal device during theperiod of time for which no terminal device specific data was scheduledfor transmission to the terminal device in response to signallingreceived from the terminal device during this period.

In accordance with some embodiments the base station is configured toreceive a channel quality indicator, CQI, from the terminal device onexpiry of the period of time for which no terminal device specific datawas transmitted to the terminal device.

According to a third aspect of the invention there is provided awireless telecommunications system comprising the base station of thesecond aspect of the invention and the terminal device for which thetype of terminal device specific data are not scheduled for transmissionduring the period of time.

According to a fourth aspect of the invention there is provided a methodof operating a terminal device in a wireless telecommunications systememploying a radio interface including an arrangement of downlinkreference symbols, the method comprising: monitoring reference symbolstransmitted by a base station; identifying that transmission by the basestation of at least one reference symbol from the arrangement ofdownlink reference symbols is suppressed; determining a period of timefor which to enter a reduced activity mode based on the identified atleast one reference symbol for which transmission is suppressed; andinitiating the reduced activity mode for the determined period of time.

In accordance with some embodiments the determined period of time forwhich to enter the reduced activity mode is based on an associationbetween different ones of the reference symbols and different potentialperiods of time for entering the reduced activity mode.

In accordance with some embodiments the association between differentones of the reference symbols and different potential periods of timefor entering the reduced activity mode is pre-defined for the wirelesstelecommunications system.

In accordance with some embodiments the association between differentones of the reference symbols and different potential periods of timefor entering the reduced activity mode is communicated to the terminaldevice from the base station.

In accordance with some embodiments the association between differentones of the reference symbols and different potential periods of timefor entering the reduced activity mode is defined in a look-up table.

In accordance with some embodiments the at least one reference symbolfor which transmission is suppressed comprises more than one referencesymbol and the period of time for entering the reduced activity mode isdetermined according to a mapping between different combinations ofreference symbols and a plurality of potential periods of time forentering the reduced activity mode.

In accordance with some embodiments the reference symbols are receivedon transmissions from multiple antenna ports of the base station and thedetermined period of time for entering the reduced activity mode isbased on which antenna port is associated with the at least onereference symbol for which transmission is suppressed.

In accordance with some embodiments the reference symbols comprisecell-specific reference symbols and/or terminal device specificreference symbols and/or demodulation reference symbols and/or channelstate information reference symbols and/or positioning referencesymbols.

In accordance with some embodiments a start time for the period of timefor entering the reduced activity mode relative to a time at which thetransmission of the at least one reference symbol is suppressed is alsobased on the at least one reference symbol for which transmission issuppressed.

In accordance with some embodiments the method further comprisesdetermining at least one further period of time for which to enter areduced activity mode based on the identified at least one referencesymbol for which transmission is suppressed.

In accordance with some embodiments the determined period of time and atleast one further period of time follow a pattern defined according tothe at least one reference symbol for which transmission is suppressed.

In accordance with some embodiments the method further comprisesdetermining to enter the reduced activity mode for a period of timebased on a correspondence between an identifier for the terminal deviceand an identity associated with the at least one reference symbol forwhich transmission is suppressed.

In accordance with some embodiments the identity associated with the atleast one reference symbol for which transmission is suppressed uniquelyidentifies the terminal device.

In accordance with some embodiments the identity associated with the atleast one reference symbol for which transmission is suppressedidentifies a group of terminal devices of which the terminal device is amember.

In accordance with some embodiments an association between the terminaldevice and the group of terminal devices of which the terminal device isa member is established by signalling between the base station and theterminal device.

In accordance with some embodiments an association between the terminaldevice and the group of terminal devices of which the terminal device isa member is pre-defined for the wireless telecommunications system.

In accordance with some embodiments the at least one reference symbol isidentified as being suppressed based on it not being received or beingreceived with less power than other reference symbols.

In accordance with some embodiments the radio interface has a downlinkradio frame structure comprising radio subframes.

In accordance with some embodiments the period of time corresponds witha number of subframes starting at an offset defined relative to asubframe in which a reference symbol is suppressed.

In accordance with some embodiments the at least one reference symbolfor which transmission is suppressed comprises at least one referencesymbol in each one of more than one subframe.

In accordance with some embodiments the radio interface comprises aplurality of Orthogonal Frequency Division Multiplexing, OFDM,sub-carriers spanning a system frequency bandwidth, and wherein theradio interface supports a first carrier for communicating with a firstclass of terminal device using a first group of the OFDM sub-carriersdistributed across the system frequency bandwidth, and a second carrierfor communicating with a second class of terminal device on a secondgroup of the OFDM sub-carriers distributed across a restricted frequencybandwidth, wherein the restricted frequency bandwidth is narrower thanand within the system frequency bandwidth, and the terminal device is aterminal device of the second class operating on the second carrier.

In accordance with some embodiments the method further comprisesderiving further information communicated from the base station to theterminal device based on the at least one reference symbol for whichtransmission is suppressed.

In accordance with some embodiments the method further comprises theterminal device transmitting signalling to the base station during thereduced activity mode to request resources for subsequent communicationsbetween the base station and the terminal device.

In accordance with some embodiments the method further comprises theterminal device transmitting a channel quality indicator, CQI, to thebase station on exit from the reduced activity mode.

In accordance with some embodiments the reduced activity mode is a modein which the terminal device is configured to decode fewer transmissionsfrom the base station than when the terminal device is not in thereduced activity mode.

In accordance with some embodiments the terminal device continues todecode at least one of synchronisation information and/or systeminformation and/or reference symbols when in the reduced activity mode.

In accordance with some embodiments the terminal device is amachine-type communication, MTC, terminal device.

In accordance with a fifth aspect of the invention there is provided aterminal device for use in a wireless telecommunications systememploying a radio interface including an arrangement of downlinkreference symbols, the terminal device comprising: a monitoring unit formonitoring reference symbols transmitted by a base station; anidentifying unit for identifying that transmission by the base stationat least one reference symbol from the arrangement of downlink referencesymbols is suppressed; a determining unit for determining a period oftime for which to enter a reduced activity mode based on the identifiedat least one reference symbol for which transmission is suppressed; andan initiating unit for initiating the reduced activity mode for thedetermined period of time.

In accordance with some embodiments the terminal device is configuredsuch that the determined period of time for which to enter the reducedactivity mode is based on an association between different ones of thereference symbols and different potential periods of time for enteringthe reduced activity mode.

In accordance with some embodiments the association between differentones of the reference symbols and different potential periods of timefor entering the reduced activity mode is pre-defined for the wirelesstelecommunications system.

In accordance with some embodiments the association between differentones of the reference symbols and different potential periods of timefor entering the reduced activity mode is communicated to the terminaldevice from the base station.

In accordance with some embodiments the association between differentones of the reference symbols and different potential periods of timefor entering the reduced activity mode is defined in a look-up table.

In accordance with some embodiments the at least one reference symbolfor which transmission is suppressed comprises more than one referencesymbol and wherein the terminal device is configured such that theperiod of time for entering the reduced activity mode is determinedaccording to a mapping between different combinations of referencesymbols and a plurality of potential periods of time for entering thereduced activity mode.

In accordance with some embodiments the terminal device is configured toreceive the reference symbols on transmissions from multiple antennaports of the base station and to determine the period of time forentering the reduced activity mode based on which antenna port isassociated with the at least one reference symbol for which transmissionis suppressed.

In accordance with some embodiments the reference symbols comprisecell-specific reference symbols and/or terminal device specificreference symbols and/or demodulation reference symbols and/or channelstate information reference symbols and/or positioning referencesymbols.

In accordance with some embodiments the terminal device is configuredsuch that a start time for a period of time for entering the reducedactivity mode relative to a time at which the transmission of the atleast one reference symbol is suppressed is also determined based on theat least one reference symbol for which transmission is suppressed.

In accordance with some embodiments the terminal device is configured todetermine at least one further period of time for which to enter areduced activity mode based on an identified at least one referencesymbol for which transmission is suppressed.

In accordance with some embodiments the determined period of time and atleast one further period of time follow a pattern defined according tothe at least one reference symbol for which transmission is suppressed.

In accordance with some embodiments the terminal device is configured todetermine to enter the reduced activity mode for a period of time basedon a correspondence between an identifier for the terminal device and anidentity associated with the at least one reference symbol for whichtransmission is suppressed.

In accordance with some embodiments the identity associated with the atleast one reference symbol for which transmission is suppressed uniquelyidentifies the terminal device.

In accordance with some embodiments the identity associated with the atleast one reference symbol for which transmission is suppressedidentifies a group of terminal devices of which the terminal device is amember.

In accordance with some embodiments the terminal device is configuredsuch that an association between the terminal device and the group ofterminal devices of which the terminal device is a member is establishedby signalling between the base station and the terminal device.

In accordance with some embodiments an association between the terminaldevice and the group of terminal devices of which the terminal device isa member is pre-defined for the wireless telecommunications system.

In accordance with some embodiments the terminal device is configuredsuch that the at least one reference symbol is identified as beingsuppressed based on it not being received or being received with lesspower than other reference symbols.

In accordance with some embodiments the radio interface has a downlinkradio frame structure comprising radio subframes.

In accordance with some embodiments the period of time corresponds witha number of subframes starting at an offset defined relative to asubframe in which a reference symbol is suppressed.

In accordance with some embodiments the at least one reference symbolfor which transmission is suppressed comprises at least one referencesymbol in each one of more than one subframe.

In accordance with some embodiments the radio interface comprises aplurality of Orthogonal Frequency Division Multiplexing, OFDM,sub-carriers spanning a system frequency bandwidth, and wherein theradio interface supports a first carrier for communicating with a firstclass of terminal device using a first group of the OFDM sub-carriersdistributed across the system frequency bandwidth, and a second carrierfor communicating with a second class of terminal device on a secondgroup of the OFDM sub-carriers distributed across a restricted frequencybandwidth, wherein the restricted frequency bandwidth is narrower thanand within the system frequency bandwidth, and the terminal device is aterminal device of the second class operating on the second carrier.

In accordance with some embodiments the terminal device is configured toderive further information communicated by the base station based on theat least one reference symbol for which transmission is suppressed.

In accordance with some embodiments the terminal device is configured totransmit signalling to the base station during the reduced activity modeto request resources for subsequent communications between the basestation and the terminal device.

In accordance with some embodiments the terminal device is configured totransmit a channel quality indicator, CQI, to the base station on exitfrom the reduced activity mode.

In accordance with some embodiments the reduced activity mode is a modein which the terminal device is configured to decode fewer transmissionsfrom the base station than when the terminal device is not in thereduced activity mode.

In accordance with some embodiments the terminal device is configured tocontinue to decode at least one of synchronisation information and/orsystem information and/or reference symbols when in the reduced activitymode.

In accordance with some embodiments the terminal device is amachine-type communication, MTC, terminal device.

According to a sixth aspect of the invention there is provided awireless telecommunications system comprising the terminal device of thefifth aspect of the invention and a base station.

It will be appreciated that features and aspects of the inventiondescribed above in relation to the first and other aspects of theinvention are equally applicable and may be combined with embodiments ofthe invention according to the different aspects of the invention asappropriate, and not just in the specific combinations described above.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present invention will now be described by way ofexample only with reference to the accompanying drawings where likeparts are provided with corresponding reference numerals and in which:

FIG. 1 provides a schematic diagram illustrating an example of aconventional mobile telecommunication network;

FIG. 2 provides a schematic diagram illustrating a conventional LTEradio frame;

FIG. 3 provides a schematic diagram illustrating an example of aconventional LTE downlink radio subframe;

FIG. 4 provides a schematic diagram illustrating a conventional LTE“camp-on” procedure;

FIG. 5 provides a schematic diagram illustrating an LTE downlink radiosubframe in which a virtual carrier has been inserted in accordance withan embodiment of the invention;

FIG. 6 provides a schematic diagram illustrating an adapted LTE“camp-on” procedure for camping on to a virtual carrier;

FIG. 7 provides a schematic diagram illustrating LTE downlink radiosubframes in accordance with an embodiment of the present invention;

FIG. 8 provides a schematic diagram illustrating a physical broadcastchannel (PBCH);

FIG. 9 provides a schematic diagram illustrating an LTE downlink radiosubframe in accordance with an embodiment of the present invention;

FIG. 10 provides a schematic diagram illustrating an LTE downlink radiosubframe in which a virtual carrier has been inserted in accordance withan embodiment of the invention;

FIGS. 11A to 11D provide schematic diagrams illustrating positioning oflocation signals within a LTE downlink subframe according to embodimentsof the present invention;

FIG. 12 provides a schematic diagram illustrating a group of subframesin which two virtual carriers change location within a host carrier bandaccording to an embodiment of the present invention;

FIGS. 13A to 13C provide schematic diagrams illustrating LTE uplinksubframes in which an uplink virtual carrier has been inserted inaccordance with an embodiment of the present invention;

FIG. 14 provides a schematic diagram showing part of an adapted LTEmobile telecommunication network arranged in accordance with an exampleof the present invention;

FIG. 15A schematically represents an example allocation of transmissionresources between a host and virtual carrier in a LTE mobiletelecommunication network arranged according to an embodiment of theinvention;

FIG. 15B schematically represents an example allocation of transmissionresources for a host carrier in a LTE mobile telecommunication networkarranged according to an embodiment of the invention;

FIG. 15C schematically represents an example allocation of transmissionresources for a virtual carrier in a LTE mobile telecommunicationnetwork arranged according to an embodiment of the invention;

FIG. 16 schematically shows a mobile telecommunication networkarchitecture according to an embodiment of the invention;

FIG. 17 schematically represents locations for cell-specific referencesymbols in a portion of a downlink radio subframe;

FIG. 18 schematically shows a correspondence between differentcell-specific reference symbols and different potential periods of timefor which a terminal device may be controlled to enter a reducedactivity state in accordance with an embodiment of the invention bysuppression of transmission of the corresponding reference symbol;

FIG. 19 is a ladder-type diagram schematically showing for somesignalling steps between a base station and a terminal device inaccordance with an embodiment of the invention; and

FIG. 20 is a flow diagram schematically representing processing in aterminal device in accordance with an embodiment of the invention.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Embodiments of the invention may in particular be employed within thecontext of what might be termed “virtual carriers” operating within abandwidth of a “host carriers”. The concepts of virtual carriers aredescribed in co-pending UK patent applications numbered GB 1101970.0[2], GB 1101981.7 [3], GB 1101966.8 [4], GB 1101983.3 [5], GB 1101853.8[6], GB 1101982.5 [7], GB 1101980.9 [8], GB 1101972.6 [9], GB 1121767.6[10] and GB 1121766.8 [11] the contents of which are incorporated hereinby reference. The reader is referred to these co-pending applicationsfor more details, but for ease of reference an overview of the conceptof virtual carriers is also provided here.

Conventional Network

FIG. 1 provides a schematic diagram illustrating some basicfunctionality of a conventional mobile telecommunications network.

The network includes a plurality of base stations 101 connected to acore network 102. Each base station provides a coverage area 103 (i.e. acell) within which data can be communicated to and from terminal devices104. Data is transmitted from base stations 101 to terminal devices 104within their respective coverage areas 103 via a radio downlink. Data istransmitted from terminal devices 104 to the base stations 101 via aradio uplink. The core network 102 routes data to and from the terminaldevices 104 via the respective base stations 101 and provides functionssuch as authentication, mobility management, charging and so on.

Mobile telecommunications systems such as those arranged in accordancewith the 3GPP defined Long Term Evolution (LTE) architecture use anorthogonal frequency division multiplex (OFDM) based interface for theradio downlink (so-called OFDMA) and the radio uplink (so-calledSC-FDMA). FIG. 2 shows a schematic diagram illustrating an OFDM basedLTE downlink radio frame 201. The LTE downlink radio frame istransmitted from an LTE base station (known as an enhanced Node B) andlasts 10 ms. The downlink radio frame comprises ten subframes, eachsubframe lasting 1 ms. A primary synchronisation signal (PSS) and asecondary synchronisation signal (SSS) are transmitted in the first andsixth subframes of the LTE frame. A primary broadcast channel (PBCH) istransmitted in the first subframe of the LTE frame. The PSS, SSS andPBCH are discussed in more detail below.

FIG. 3 is a schematic diagram of a grid which illustrates the structureof an example conventional downlink LTE subframe. The subframe comprisesa predetermined number of symbols which are transmitted over a 1 msperiod. Each symbol comprises a predetermined number of orthogonalsub-carriers distributed across the bandwidth of the downlink radiocarrier.

The example subframe shown in FIG. 3 comprises 14 symbols and 1200sub-carriers spread across a 20 MHz bandwidth. The smallest allocationof user data for transmission in LTE is a resource block comprisingtwelve sub-carriers transmitted over one slot (0.5 subframe). Forclarity, in FIG. 3, each individual resource element is not shown,instead each individual box in the subframe grid corresponds to twelvesub-carriers transmitted on one symbol.

FIG. 3 shows in hatching resource allocations for four LTE terminals340, 341, 342, 343. For example, the resource allocation 342 for a firstLTE terminal (UE 1) extends over five blocks of twelve sub-carriers(i.e. 60 sub-carriers), the resource allocation 343 for a second LTEterminal (UE2) extends over six blocks of twelve sub-carriers and so on.

Control channel data is transmitted in a control region 300 (indicatedby dotted-shading in FIG. 3) of the subframe comprising the first nsymbols of the subframe where n can vary between one and three symbolsfor channel bandwidths of 3 MHz or greater and where n can vary betweentwo and four symbols for channel bandwidths of 1.4 MHz. For the sake ofproviding a concrete example, the following description relates to hostcarriers with a channel bandwidth of 3 MHz or greater so the maximumvalue of n will be 3. The data transmitted in the control region 300includes data transmitted on the physical downlink control channel(PDCCH), the physical control format indicator channel (PCFICH) and thephysical HARQ indicator channel (PHICH).

PDCCH contains control data indicating which sub-carriers on whichsymbols of the subframe have been allocated to specific LTE terminals.Thus, the PDCCH data transmitted in the control region 300 of thesubframe shown in FIG. 3 would indicate that UE1 has been allocated theblock of resources identified by reference numeral 342, that UE2 hasbeen allocated the block of resources identified by reference numeral343, and so on.

PCFICH contains control data indicating the size of the control region(i.e. between one and three symbols).

PHICH contains HARQ (Hybrid Automatic Request) data indicating whetheror not previously transmitted uplink data has been successfully receivedby the network.

Symbols in a central band 310 of the time-frequency resource grid areused for the transmission of information including the primarysynchronisation signal (PSS), the secondary synchronisation signal (SSS)and the physical broadcast channel (PBCH). This central band 310 istypically 72 sub-carriers wide (corresponding to a transmissionbandwidth of 1.08 MHz). The PSS and SSS are synchronisation signals thatonce detected allow an LTE terminal device to achieve framesynchronisation and determine the cell identity of the enhanced Node Btransmitting the downlink signal. The PBCH carries information about thecell, comprising a master information block (MIB) that includesparameters that LTE terminals use to properly access the cell. Datatransmitted to individual LTE terminals on the physical downlink sharedchannel (PDSCH) can be transmitted in other resource elements of thesubframe. Further explanation of these channels is provided below.

FIG. 3 also shows a region of PDSCH containing system information andextending over a bandwidth of R₃₄₄. A conventional LTE frame will alsoinclude reference signals which are discussed further below but notshown in FIG. 3 in the interests of clarity.

The number of sub-carriers in an LTE channel can vary depending on theconfiguration of the transmission network. Typically this variation isfrom 72 sub carriers contained within a 1.4 MHz channel bandwidth to1200 sub-carriers contained within a 20 MHz channel bandwidth (asschematically shown in FIG. 3). As is known in the art, data transmittedon the PDCCH, PCFICH and PHICH is typically distributed on thesub-carriers across the entire bandwidth of the subframe to provide forfrequency diversity. Therefore a conventional LTE terminal must be ableto receive the entire channel bandwidth in order to receive and decodethe control region.

FIG. 4 illustrates an LTE “camp-on” process, that is, the processfollowed by a terminal so that it can decode downlink transmissionswhich are sent by a base station via a downlink channel. Using thisprocess, the terminal can identify the parts of the transmissions thatinclude system information for the cell and thus decode configurationinformation for the cell.

As can be seen in FIG. 4, in a conventional LTE camp-on procedure, theterminal first synchronizes with the base station (step 400) using thePSS and SSS in the centre band and then decodes the PBCH (step 401).Once the terminal has performed steps 400 and 401, it is synchronizedwith the base station.

For each subframe, the terminal then decodes the PCFICH which isdistributed across the entire bandwidth of carrier 320 (step 402). Asdiscussed above, an LTE downlink carrier can be up to 20 MHz wide (1200sub-carriers) and an LTE terminal therefore has to have the capabilityto receive and decode transmissions on a 20 MHz bandwidth in order todecode the PCFICH. At the PCFICH decoding stage, with a 20 MHz carrierband, the terminal operates at a much larger bandwidth (bandwidth ofR₃₂₀) than during steps 400 and 401 (bandwidth of R₃₁₀) relating tosynchronization and PBCH decoding.

The terminal then ascertains the PHICH locations (step 403) and decodesthe PDCCH (step 404), in particular for identifying system informationtransmissions and for identifying its resource allocations. The resourceallocations are used by the terminal to locate system information and tolocate its data in the PDSCH as well as to be informed of anytransmission resources it has been granted on PUSCH. Both systeminformation and UE-specific resource allocations are transmitted onPDSCH and scheduled within the carrier band 320. Steps 403 and 404 alsorequire the terminal to operate on the entire bandwidth R320 of thecarrier band.

At steps 402 to 404, the terminal decodes information contained in thecontrol region 300 of a subframe. As explained above, in LTE, the threecontrol channels mentioned above (PCFICH, PHICH and PDCCH) can be foundacross the control region 300 of the carrier where the control regionextends over the range R₃₂₀ and occupies the first one, two or threeOFDM symbols of each subframe as discussed above. In a subframe,typically the control channels do not use all the resource elementswithin the control region 300, but they are scattered across the entireregion, such that a LTE terminal has to be able to simultaneouslyreceive the entire control region 300 for decoding each of the threecontrol channels.

The terminal can then decode the PDSCH (step 405) which contains systeminformation or data transmitted for this terminal.

As explained above, in an LTE subframe the PDSCH generally occupiesgroups of resource elements which are neither in the control region norin the resource elements occupied by PSS, SSS or PBCH. The data in theblocks of resource elements 340, 341, 342, 343 allocated to thedifferent mobile communication terminals (UEs) shown in FIG. 3 have asmaller bandwidth than the bandwidth of the entire carrier, although todecode these blocks a terminal first receives the PDCCH spread acrossthe frequency range R₃₂₀ to determine if the PDCCH indicates that aPDSCH resource is allocated to the UE and should be decoded. Once a UEhas received the entire subframe, it can then decode the PDSCH in therelevant frequency range (if any) indicated by the PDCCH. So forexample, UE 1 discussed above decodes the whole control region 300 andthen the data in the resource block 342.

Virtual Downlink Carrier

Certain classes of devices, such as MTC devices (e.g. semi-autonomous orautonomous wireless communication devices such as smart meters asdiscussed above), support communication applications that arecharacterised by the transmission of small amounts of data at relativelyinfrequent intervals and can thus be considerably less complex thanconventional LTE terminals. In many scenarios, providing low capabilityterminals such as those with a conventional high-performance LTEreceiver unit capable of receiving and processing data from an LTEdownlink frame across the full carrier bandwidth can be overly complexfor a device which only needs to communicate small amounts of data. Thismay therefore limit the practicality of a widespread deployment of lowcapability MTC type devices in an LTE network. It is preferable insteadto provide low capability terminals such as MTC devices with a simplerreceiver unit which is more proportionate with the amount of data likelyto be transmitted to the terminal. As set out below, in accordance withexamples of the present invention a “virtual carrier” is provided withinthe transmission resources of a conventional OFDM type downlink carrier(i.e. a “host carrier”). Unlike data transmitted on a conventional OFDMtype downlink carrier, data transmitted on the virtual carrier can bereceived and decoded without needing to process the full bandwidth ofthe downlink host OFDM carrier. Accordingly, data transmitted on thevirtual carrier can be received and decoded using a reduced complexityreceiver unit.

FIG. 5 provides a schematic diagram illustrating an LTE downlinksubframe which includes a virtual carrier inserted in a host carrier inaccordance with an example of the present invention.

In keeping with a conventional LTE downlink subframe, the first nsymbols (n is three in FIG. 5) form the control region 300 which isreserved for the transmission of downlink control data such as datatransmitted on the PDCCH. However, as can be seen from FIG. 5, outsideof the control region 300 the LTE downlink subframe includes a group ofresource elements positioned in this example below the central band 310which form a virtual carrier 501. As explained further below, thevirtual carrier 501 is adapted so that data transmitted on the virtualcarrier 501 can be treated as logically distinct from data transmittedin the remaining parts of the host carrier and can be decoded withoutdecoding all the control data from the control region 300. Although FIG.5 shows the virtual carrier occupying frequency resources below thecentre band, in general the virtual carrier can occupy other frequencyresources, for example, above the centre band or including the centreband. If the virtual carrier is configured to overlap any resources usedby the PSS, SSS or PBCH of the host carrier, or any other signaltransmitted by the host carrier that a terminal device operating on thehost carrier would require for correct operation and expect to find in aknown pre-determined location, the signals on the virtual carrier can bearranged such that these aspects of the host carrier signal aremaintained.

As can be seen from FIG. 5, data transmitted on the virtual carrier 501is transmitted across a limited bandwidth. This might be any suitablebandwidth smaller than that of the host carrier. In the example shown inFIG. 5 the virtual carrier is transmitted across a bandwidth comprising12 blocks of 12 sub-carriers (i.e. 144 sub-carriers), which isequivalent to a 2.16 MHz transmission bandwidth. Accordingly, a terminalusing the virtual carrier need only be equipped with a receiver capableof receiving and processing data transmitted over a bandwidth of 2.16MHz. This enables low capability terminals (for example MTC typeterminals) to be provided with simplified receiver units yet still beable to operate within an OFDM type communication network which, asexplained above, conventionally requires terminals to be equipped withreceivers capable of receiving and processing an OFDM signal across theentire bandwidth of the signal.

As explained above, in OFDM-based mobile communication systems such asLTE, downlink data is dynamically assigned to be transmitted ondifferent sub-carriers on a subframe by subframe basis. Accordingly, inevery subframe the network signals which sub-carriers on which symbolscontain data relevant to which terminals (i.e. downlink allocationsignalling).

As can be seen from FIG. 3, in a conventional downlink LTE subframe thisinformation is transmitted on the PDCCH during the first symbol orsymbols of the subframe. However, as previously explained, theinformation transmitted in the PDCCH is spread across the entirebandwidth of the subframe and therefore cannot be received by a mobilecommunication terminal with a simplified receiver unit capable only ofreceiving the reduced bandwidth virtual carrier.

Accordingly, as can be seen in FIG. 5, the final symbols of the virtualcarrier can be reserved as a control region 502 for the virtual carrierfor the transmission of control data indicating which resource elementsof the virtual carrier 501 have been allocated to user equipment (UEs)using the virtual carrier. In some examples the number of symbolscomprising the virtual carrier control region 502 might be fixed, forexample three symbols. In other examples the virtual carrier controlregion 502 can vary in size, for example between one and three symbols,as with the control region 300.

The virtual carrier control region can be located at any suitableposition, for example in the first few symbols of the virtual carrier.In the example of FIG. 5 this could mean positioning the virtual carriercontrol region on the fourth, fifth and sixth symbols. However, fixingthe position of the virtual carrier control region in the final symbolsof the subframe can be useful because the position of the virtualcarrier control region will not vary in dependence on the number ofsymbols of the host carrier control region 300. This can help simplifythe processing undertaken by mobile communication terminals receivingdata on the virtual carrier because there is no need for terminals todetermine a position of the virtual carrier control region everysubframe if it is known that it will always be positioned in the final nsymbols of the subframe.

In a further embodiment, the virtual carrier control symbols mayreference virtual carrier PDSCH transmissions in a separate subframe.

In some examples the virtual carrier may be located within the centreband 310 of the downlink subframe. This can help reduce the impact onhost carrier PDSCH resources caused by the introduction of the virtualcarrier within the host carrier bandwidth since the resources occupiedby the PSS/SSS and PBCH would be contained within the virtual carrierregion and not the remaining host carrier PDSCH region. Therefore,depending on for example the expected virtual carrier throughput, thelocation of a virtual carrier can be appropriately chosen to eitherexist inside or outside the centre band according to whether the host orvirtual carrier is chosen to bear the overhead of the PSS, SSS and PBCH.

Virtual Carrier “Camp-On” Process

As explained above, before a conventional LTE terminal can begintransmitting and receiving data in a cell, it first camps on to thecell. An adapted camp-on process can be provided for terminals using thevirtual carrier.

FIG. 6 shows a flow diagram schematically illustrating a camp-on processaccording to an example of the present invention. There are two branchesshown in FIG. 6. Different steps of the process associated with a UEintending to use the virtual carrier are shown under the general heading“virtual carrier”. The steps shown under the general heading “legacyLTE” are associated with a UE intending to use the host carrier, andthese steps correspond to the steps of FIG. 4. In this example, thefirst two steps 400, 401 of the camp-on procedure are common to both thevirtual carrier and host (legacy LTE) carrier.

The virtual carrier camp-on process is explained with reference to theexample subframe shown in FIG. 5 in which a virtual carrier with abandwidth of 144 sub-carriers is inserted within the operating bandwidthof a host carrier with a bandwidth corresponding to 1200 sub-carriers.As discussed above, a terminal having a receiver unit with anoperational bandwidth of less than that of the host carrier cannot fullydecode data in the control region of subframes of the host carrier.However, a receiver unit of a terminal having an operational bandwidthof only twelve blocks of twelve sub-carriers (i.e. 2.16 MHz) can receivecontrol and user data transmitted on this example virtual carrier 502.

As noted above, in the example of FIG. 6, the first steps 400 and 401for a virtual carrier terminal are the same as the conventional camp-onprocess shown in FIG. 4, although a virtual carrier terminal may extractadditional information from the MIB as described below. Both types ofterminals (i.e. virtual carrier terminals and host/legacy carrierterminals) can use the PSS/SSS and PBCH to synchronize with the basestation using the information carried on the 72 sub-carrier centre bandwithin the host carrier. However, where the conventional LTE terminalsthen continue with the process by performing the PCFICH decoding step402, which requires a receiver unit capable of receiving and decodingthe host carrier control region 300, a terminal camping on to the cellto receive data on the virtual carrier (which may be referred to as a“virtual carrier terminal”) performs steps 606 and 607 instead.

In a further example a separate synchronisation and PBCH functionalitycan be provided for the virtual carrier device as opposed to re-usingthe same conventional initial camp-on processes of steps 400 and 401 ofthe host carrier device.

At step 606, the virtual carrier terminal locates a virtual carrier, ifany is provided within the host carrier, using a virtualcarrier-specific step. Various examples of how this step may beperformed are discussed further below. Once the virtual carrier terminalhas located a virtual carrier, it can access information within thevirtual carrier. For example, if the virtual carrier mirrors theconventional LTE resource allocation method, the virtual carrierterminal may proceed to decode control portions within the virtualcarrier, which can, for example, indicate which resource elements withinthe virtual carrier have been allocated for a specific virtual carrierterminal or for system information. For example, FIG. 7 shows the blocksof resource elements 350 to 352 within virtual carrier 330 that havebeen allocated for the subframe SF2. However, there is no requirementfor the virtual carrier terminal to follow or mirror the conventionalLTE process (e.g. steps 402-404) and these steps may for example beimplemented very differently for a virtual carrier camp-on process.

Regardless of the virtual carrier terminal following a LTE-like step ora different type of step when performing step 607, the virtual carrierterminal can then decode the allocated resource elements at step 608 andthereby receive data transmitted by the base station broadcasting thevirtual carrier. The data decoded in step 608 may include, for example,the remainder of the system information containing details of thenetwork configuration.

Even though the virtual carrier terminal does not have the bandwidthcapabilities to decode and receive downlink data if it was transmittedin the host carrier using conventional LTE, it can still access avirtual carrier within the host carrier having a limited bandwidthwhilst re-using the initial LTE steps. Step 608 may also be implementedin a LTE-like manner or in a different manner. For example, multiplevirtual carrier terminals may share a virtual carrier and have grantsallocated to manage the virtual carrier sharing as shown in SF2 in FIG.7, or, in another example, a virtual carrier terminal may have theentire virtual carrier allocated for its own downlink transmissions, orthe virtual carrier may be entirely allocated to a virtual carrierterminal for a certain number of subframe only, etc.

There is thus a large degree of flexibility provided for the virtualcarrier camp-on process. There is, for example, the ability to adjust abalance between re-using or mirroring conventional LTE steps orprocesses, thereby reducing the terminal complexity and the need toimplement new elements, and adding new virtual carrier specific aspectsor implementations, thereby potentially optimizing the use ofnarrow-band virtual carriers, as LTE has been designed with thelarger-band host carriers in mind.

Downlink Virtual Carrier Detection

As discussed above, the virtual carrier terminal should locate (withinthe time-frequency resource grid of the host carrier) the virtualcarrier before it can receive and decode transmissions on the virtualcarrier. Several alternatives are available for the virtual carrierpresence and location determination, which can be implemented separatelyor in combination. Some of these options are discussed below.

To facilitate the virtual carrier detection, the virtual carrierlocation information may be provided to the virtual carrier terminalsuch that it can locate the virtual carrier, if any exists, more easily.For example, such location information may comprise an indication thatone or more virtual carriers are provided within the host carrier, orthat the host carrier does not currently provide any virtual carrier. Itmay also comprise an indication of the virtual carrier's bandwidth, forexample in MHz or blocks of resource elements. Alternatively, or incombination, the virtual carrier location information may comprise thevirtual carrier's centre frequency and bandwidth, thereby giving thevirtual carrier terminal the location and bandwidth of any activevirtual carrier. In the event the virtual carrier is to be found at adifferent frequency position in each subframe, according, for example,to a pseudo-random hopping algorithm, the location information can, forexample, indicate a pseudo random parameter. Such parameters may includea starting frame and parameters used for the pseudo-random algorithm.Using these pseudo-random parameters, the virtual carrier terminal canthen know where the virtual carrier can be found for any subframe.

On implementation feature associated with little change to the virtualcarrier terminal (as compared with a conventional LTE terminal) would beto include location information for the virtual carrier within the PBCH,which already carries the Master Information Block, or MIB in the hostcarrier centre band. As shown in FIG. 8, the MIB consists of 24 bits (3bits to indicate DL bandwidth, 8 bits to indicate the System FrameNumber or SFN, and 3 bits regarding the PHICH configuration). The MIBtherefore comprises 10 spare bits that can be used to carry locationinformation in respect of one or more virtual carriers. For example,FIG. 9 shows an example where the PBCH includes the MIB and locationinformation (“LI”) for pointing any virtual carrier terminal to avirtual carrier.

Alternatively, virtual carrier location information could be provided inthe centre band, outside of the PBCH. It can for example be alwaysprovided after and adjacent to the PBCH. By providing the locationinformation in the centre band but outside of the PBCH, the conventionalPBCH is not modified for the purpose of using virtual carriers, but avirtual carrier terminal can easily find the location information inorder to detect the virtual carrier, if any.

The virtual carrier location information, if provided, can be providedelsewhere in the host carrier, but it may be advantageous to provide itin the centre band, for example because a virtual carrier terminal mayconfigure its receiver to operate on the centre band and the virtualcarrier terminal then does not need to adjust its receiver settings forfinding the location information.

Depending on the amount of virtual carrier location informationprovided, the virtual carrier terminal can either adjust its receiver toreceive the virtual carrier transmissions, or it may require furtherlocation information before it can do so.

If for example, the virtual carrier terminal was provided with locationinformation indicating a virtual carrier presence and/or a virtualcarrier bandwidth but not indicating any details as to the exact virtualcarrier frequency range, or if the virtual carrier terminal was notprovided with any location information, the virtual carrier terminalcould then scan the host carrier for a virtual carrier (e.g. performinga so-called blind search process). Scanning the host carrier for avirtual carrier can be based on different approaches, some of which willbe presented below.

According to a first approach, a virtual carrier might only be insertedin certain pre-determined locations, as illustrated for example in FIG.10 for a four-location example. The virtual carrier terminal then scansthe four locations L1-L4 for any virtual carrier. If and when thevirtual carrier terminal detects a virtual carrier, it can then“camp-on” the virtual carrier to receive downlink data as describedabove. In this approach, the virtual carrier terminal may be providedwith the possible virtual carrier locations in advance, for example theymay be stored as a network-specific setting in an internal memory.Detection of a virtual carrier could be accomplished by seeking todecode a particular physical channel on the virtual carrier. Thesuccessful decoding of such a channel, indicated for example by asuccessful cyclic redundancy check (CRC) on decoded data, would indicatethe successful location of the virtual carrier

According to a second approach, the virtual carrier may include locationsignals such that a virtual carrier terminal scanning the host carriercan detect such signals to identify the presence of a virtual carrier.Examples of possible location signals are illustrated in FIGS. 11A to11D. In the examples of FIGS. 11A to 11C, the virtual carrier regularlysends an arbitrary location signal such that a terminal scanning afrequency range where the location signal is would detect this signal.An “arbitrary” signal is intended here to include any signal that doesnot carry any information as such, or is not meant to be interpreted,but merely includes a specific signal or pattern that a virtual carrierterminal can detect. This can for example be a series of positive bitsacross the entire location signal, an alternation of 0 and 1 across thelocation signal, or any other suitable arbitrary signal. It isnoteworthy that the location signal may be made of adjacent blocks ofresource elements or may be formed of non-adjacent blocks. For example,it may be located at every other block of resource elements at the “top”(i.e. upper frequency limit) of the virtual carrier.

In the example of FIG. 11A, the location signal 353 extends across therange R₃₃₀ of the virtual carrier 330 and is always found at the sameposition in the virtual carrier within a subframe. If the virtualcarrier terminal knows where to look for a location signal in a virtualcarrier subframe, it can then simplify its scanning process by onlyscanning this position within a subframe for a location signal. FIG. 11Bshows a similar example where every subframe includes a location signal354 comprising two parts: one at the top corner and one at the bottomcorner of the virtual carrier subframe, at the end of this subframe.Such a location signal may be useful if, for example, the virtualcarrier terminal does not know the bandwidth of the virtual carrier inadvance as it can facilitate a clear detection of the top and bottomfrequency edges of the virtual carrier band.

In the example of FIG. 11C, a location signal 355 is provided in a firstsubframe SF1, but not in a second subframe SF2. The location signal canfor example be provided every two subframes. The frequency of thelocation signals can be chosen to adjust a balance between reducingscanning time and reducing overhead. In other words, the more often thelocation signal is provided, the less long it takes a terminal to detecta virtual carrier but the more overhead there is.

In the example of FIG. 11D, a location signal is provided where thislocation signal is not an arbitrary signal as in FIGS. 11A to 11C, butis a signal that includes information for virtual carrier terminals. Thevirtual carrier terminals can detect this signal when they scan for avirtual carrier and the signal may include information in respect of,for example, the virtual carrier bandwidth or any other virtualcarrier-related information (location or non-location information). Whendetecting this signal, the virtual carrier terminal can thereby detectthe presence and location of the virtual carrier. As shown in FIG. 11D,the location signal can, like an arbitrary location signal, be found atdifferent locations within the subframe, and the location may vary on aper-subframe basis.

Dynamic Variation of Control Region Size of Host Carrier

As explained above, in LTE the number of symbols that make up thecontrol region of a downlink subframe varies dynamically depending onthe quantity of control data that needs to be transmitted. Typically,this variation is between one and three symbols. As will be understoodwith reference to FIG. 5, variation in the width of the host carriercontrol region will cause a corresponding variance in the number ofsymbols available for the virtual carrier. For example, as can be seenin FIG. 5, when the control region is three symbols in length and thereare 14 symbols in the subframe, the virtual carrier is eleven symbolslong. However, if in the next subframe the control region of the hostcarrier were reduced to one symbol, there would be thirteen symbolsavailable for the virtual carrier in that subframe.

When a virtual carrier is inserted into a LTE host carrier, mobilecommunication terminals receiving data on the virtual carrier need to beable to determine the number of symbols in the control region of eachhost carrier subframe to determine the number of symbols in the virtualcarrier in that subframe if they are to be able to use all availablesymbols that are not used by the host carrier control region.

Conventionally, the number of symbols forming the control region issignalled in the first symbol of every subframe in the PCFICH. However,the PCFICH is typically distributed across the entire bandwidth of thedownlink LTE subframe and is therefore transmitted on sub-carriers whichvirtual carrier terminals capable only of receiving the virtual carriercannot receive. Accordingly, in one embodiment, any symbols across whichthe control region could possibly extend are predefined as null symbolson the virtual carrier, i.e. the length of the virtual sub-carrier isset at (m-n) symbols, where m is the total number of symbols in asubframe and n is the maximum number of symbols of the control region.Thus, resource elements are never allocated for downlink datatransmission on the virtual carrier during the first n symbols of anygiven subframe.

Although this embodiment is simple to implement it will be spectrallyinefficient because during subframes when the control region of the hostcarrier has fewer than the maximum number of symbols, there will beunused symbols in the virtual carrier.

In another embodiment, the number of symbols in the control region ofthe host carrier is explicitly signalled in the virtual carrier itself.Once the number of symbols in the control region of the host carrier isknown, the number of symbols in the virtual carrier can be calculated bysubtracting the total number of symbols in the subframe from thisnumber.

In one example an explicit indication of the host carrier control regionsize is given by certain information bits in the virtual carrier controlregion. In other words an explicit signalling message is inserted at apredefined position in the virtual carrier control region 502. Thispredefined position is known by each terminal adapted to receive data onthe virtual carrier.

In another example, the virtual carrier includes a predefined signal,the location of which indicates the number of symbols in the controlregion of the host carriers. For example, a predefined signal could betransmitted on one of three predetermined blocks of resource elements.When a terminal receives the subframe it scans for the predefinedsignal. If the predefined signal is found in the first block of resourceelements this indicates that the control region of the host carriercomprises one symbol; if the predefined signal is found in the secondblock of resource elements this indicates that the control region of thehost carrier comprises two symbols and if the predefined signal is foundin the third block of resource elements this indicates that the controlregion of the host carrier comprises three symbols.

In another example, the virtual carrier terminal is arranged to firstattempt to decode the virtual carrier assuming that the control regionsize of the host carrier is one symbol. If this is not successful, thevirtual carrier terminal attempts to decode the virtual carrier assumingthat the control region size of the host carrier is two and so on, untilthe virtual carrier terminal successfully decodes the virtual carrier.

Downlink Virtual Carrier Reference Signals

As is known in the art, in OFDM-based transmission systems, such as LTE,a number of sub-carriers in symbols throughout the subframes aretypically reserved for the transmission of reference signals. Asexplained further below, reference symbols play a significant role insome embodiments of the invention. However, some conventional aspects ofreference symbols are first described. Reference signals areconventionally transmitted on sub-carriers distributed throughout asubframe across the channel bandwidth and across the OFDM symbols. Thereference signals are arranged in a repeating pattern and can be used bya receiver to estimate the channel function applied to the datatransmitted on each sub-carrier using extrapolation and interpolationtechniques. These reference signals are also typically used foradditional purposes such as determining metrics for received signalpower indications, automatic frequency control metrics and automaticgain control metrics. In LTE the positions of the reference signalbearing sub-carriers within each subframe are pre-determined and knownat the transceiver of each terminal.

In a conventional LTE downlink subframes, there are a number ofdifferent reference signals, transmitted for different purposes. Oneexample is the cell-specific reference signal, broadcast to allterminals. Cell-specific reference symbols are typically inserted onevery sixth sub-carrier on each transmit antenna port on which theyoccur. Accordingly, if a virtual carrier is inserted in an LTE downlinksubframe, even if the virtual carrier has a minimum bandwidth of oneresource block (i.e. twelve sub-carriers) the virtual carrier willinclude at least some cell-specific reference signal bearingsub-carriers.

There are sufficient reference signal bearing sub-carriers provided ineach subframe such that a receiver need not accurately receive everysingle reference signal to decode the data transmitted in the subframe.However, as will be understood the more reference signals that arereceived, the better a receiver will generally be able to estimate thechannel response, and hence fewer errors will typically be introducedinto the data decoded from the subframe. Accordingly, in order topreserve compatibility with LTE communication terminals receiving dataon the host carrier, in accordance with some examples of the presentinvention, the sub-carrier positions that would contain referencesignals in a conventional LTE subframe are retained in the virtualcarrier, subject to the exceptions discussed further below in accordancewith embodiments of the invention.

As will be understood, in accordance with examples of the presentinvention, terminals arranged to receive only the virtual carrierreceive a reduced number of sub-carriers compared to conventional LTEterminals which receive each subframe across the entire bandwidth of thesubframe. As a result, the reduced capability terminals receive fewerreference signals over a narrower range of frequencies which may resultin a less accurate channel estimation being generated.

In some examples a simplified virtual carrier terminal may have a lowermobility which requires fewer reference symbols to support channelestimation. However, in some examples of the present invention thedownlink virtual carrier may include additional reference signal bearingsub-carriers to enhance the accuracy of the channel estimation that thereduced capability terminals can generate (i.e. there may be a greaterdensity of reference symbols on the virtual carrier as compared to otherregions on the host carrier).

In some examples the positions of the additional reference bearingsub-carriers are such that they are systematically interspersed withrespect to the positions of the conventional reference signal bearingsub-carriers thereby increasing the sampling frequency of the channelestimation when combined with the reference signals from the existingreference signal bearing sub-carriers. This allows an improved channelestimation of the channel to be generated by the reduced capabilityterminals across the bandwidth of the virtual carrier. In otherexamples, the positions of the additional reference bearing sub-carriersare such that they are systematically placed at the edge of thebandwidth of the virtual carrier thereby increasing the interpolationaccuracy of the virtual carrier channel estimates.

Alternative Virtual Carrier Arrangements

So far examples of the invention have been described generally in termsof a host carrier in which a single virtual carrier has been inserted asshown for example in FIG. 5. However, in some examples a host carriermay include more than one virtual carrier as shown for example in FIG.12. FIG. 12 shows an example in which two virtual carriers VC1 (330) andVC2 (331) are provided within a host carrier 320. In this example, thetwo virtual carriers change location within the host carrier bandaccording to a pseudo-random algorithm. However, in other examples, oneor both of the two virtual carriers may always be found in the samefrequency range within the host carrier frequency range and/or maychange position according to a different mechanism. In LTE, the numberof virtual carriers within a host carrier is only limited by the size ofthe host carrier. However, too many virtual carriers within the hostcarrier may unduly limit the bandwidth available for transmitting datato conventional LTE terminals and an operator may therefore decide on anumber of virtual carrier within a host carrier according to, forexample, a ratio of conventional LTE users/virtual carrier users.

In some examples the number of active virtual carriers can bedynamically adjusted such that it fits the current needs of conventionalLTE terminals and virtual carrier terminals. For example, if no virtualcarrier terminal is connected or if their access is to be intentionallylimited, the network can arrange to begin scheduling the transmission ofdata to LTE terminals within the sub-carriers previously reserved forthe virtual carrier. This process can be reversed if the number ofactive virtual carrier terminals begins to increase. In some examplesthe number of virtual carriers provided may be increased in response toan increase in the presence of virtual carrier terminals. For example ifthe number of virtual carrier terminals present in a network or area ofa network exceeds a threshold value, an additional virtual carrier isinserted in the host carrier. The network elements and/or networkoperator can thus activate or deactivate the virtual carriers wheneverappropriate.

The virtual carrier shown for example in FIG. 5 is 144 sub-carriers inbandwidth. However, in other examples a virtual carrier may be of anysize between twelve sub-carriers to 1188 sub-carriers (for a carrierwith a 1200 sub-carrier transmission bandwidth). Because in LTE thecentre band has a bandwidth of 72 sub-carriers, a virtual carrierterminal in an LTE environment preferentially has a receiver bandwidthof at least 72 sub-carriers (1.08 MHz) such that it can decode thecentre band 310, therefore a 72 sub-carrier virtual carrier may providea convenient implementation option. With a virtual carrier comprising 72sub-carriers, the virtual carrier terminal does not have to adjust thereceiver's bandwidth for camping on the virtual carrier which maytherefore reduce complexity of performing the camp-on process, but thereis no requirement to have the same bandwidth for the virtual carrier asfor the centre band and, as explained above, a virtual carrier based onLTE can be of any size between 12 to 1188 sub-carriers. For example, insome systems, a virtual carrier having a bandwidth of less than 72sub-carriers may be considered as a waste of the virtual carrierterminal's receiver resources, but from another point of view, it may beconsidered as reducing the impact of the virtual carrier on the hostcarrier by increasing the bandwidth available to conventional LTEterminals. The bandwidth of a virtual carrier can therefore be adjustedto achieve the desired balance between complexity, resource utilization,host carrier performance and requirements for virtual carrier terminals.

Uplink Transmission Frame

So far, the virtual carrier has been discussed primarily with referenceto the downlink, however in some examples a virtual carrier can also beinserted in the uplink.

In frequency division duplex (FDD) networks both the uplink and downlinkare active in all subframes, whereas in time division duplex (TDD)networks subframes can either be assigned to the uplink, to thedownlink, or further sub-divided into uplink and downlink portions.

In order to initiate a connection to a network, conventional LTEterminals make a random access request on the physical random accesschannel (PRACH). The PRACH is located in predetermined blocks ofresource elements in the uplink frame, the positions of which aresignaled to the LTE terminals in the system information signaled on thedownlink.

Additionally, when there is pending uplink data to be transmitted froman LTE terminal and the terminal does not already have any uplinkresources allocated to it, it can transmit a random access request PRACHto the base station. A decision is then made at the base station as towhich if any uplink resource is to be allocated to the terminal devicethat has made the request. Uplink resource allocations are then signaledto the LTE terminal on the physical downlink control channel (PDCCH)transmitted in the control region of the downlink subframe.

In LTE, transmissions from each terminal device are constrained tooccupy a set of contiguous resource blocks in a frame. For the physicaluplink shared channel (PUSCH) the uplink resource allocation grantreceived from the base station will indicate which set of resourceblocks to use for that transmission, where these resource blocks couldbe located anywhere within the channel bandwidth.

The first resources used by the LTE physical uplink control channel(PUCCH) are located at both the upper and lower edge of the channel,where each PUCCH transmission occupies one resource block. In the firsthalf of a subframe this resource block is located at one channel edge,and in the second half of a subframe this resource block is located atthe opposite channel edge. As more PUCCH resources are required,additional resource blocks are assigned in a sequential manner, movinginward from the channel edges. Since PUCCH signals are code divisionmultiplexed, an LTE uplink can accommodate multiple PUCCH transmissionsin the same resource block.

Virtual Uplink Carrier

In accordance with embodiments of the present invention, the virtualcarrier terminals described above can also be provided with a reducedcapability transmitter for transmitting uplink data. The virtual carrierterminals are arranged to transmit data across a reduced bandwidth. Theprovision of a reduced capability transmitter unit providescorresponding advantages to those achieved by providing a reducedcapability receiver unit with, for example, classes of devices that aremanufactured with a reduced capability for use with, for example, MTCtype applications.

In correspondence with the downlink virtual carrier, the virtual carrierterminals transmit uplink data across a reduced range of sub-carrierswithin a host carrier that has a greater bandwidth than that of thereduced bandwidth virtual carrier. This is shown in FIG. 13A. As can beseen from FIG. 13A, a group of sub-carriers in an uplink subframe form avirtual carrier 1301 within a host carrier 1302. Accordingly, thereduced bandwidth across which the virtual carrier terminals transmituplink data can be considered a virtual uplink carrier.

In order to implement the virtual uplink carrier, the base stationscheduler serving a virtual carrier ensures that all uplink resourceelements granted to virtual carrier terminals are sub-carriers that fallwithin the reduced bandwidth range of the reduced capability transmitterunits of the virtual carrier terminals. Correspondingly, the basestation scheduler serving the host carrier typically ensures that alluplink resource elements granted to host carrier terminals aresub-carriers that fall outside the set of sub-carriers occupied by thevirtual carrier terminals. However, if the schedulers for the virtualcarrier and the host carrier are implemented jointly, or have means toshare information, then the scheduler of the host carrier can assignresource elements from within the virtual carrier region to terminaldevices on the host carrier during subframes when the virtual carrierscheduler indicates that some or all of the virtual carrier resourceswill not be used by terminal devices on the virtual carrier.

If a virtual carrier uplink incorporates a physical channel that followsa similar structure and method of operation to the LTE PUCCH, whereresources for that physical channel are expected to be at the channeledges, for virtual carrier terminals these resources could be providedat the edges of the virtual carrier bandwidth and not at the edges ofthe host carrier. This is advantageous since it would ensure thatvirtual carrier uplink transmissions remain within the reduced virtualcarrier bandwidth.

Virtual Uplink Carrier Random Access

In accordance with conventional LTE techniques, it cannot be guaranteedthat the PRACH will be within the sub-carriers allocated to the virtualcarrier. In some embodiments therefore, the base station provides asecondary PRACH within the virtual uplink carrier, the location of whichcan be signaled to the virtual carrier terminals via system informationon the virtual carrier. This is shown for example in FIG. 13B in which aPRACH 1303 is located within the virtual carrier 1301. Thus, the virtualcarrier terminals send PRACH requests on the virtual carrier PRACHwithin the virtual uplink carrier. The position of the PRACH can besignaled to the virtual carrier terminals in a virtual carrier downlinksignaling channel, for example in system information on the virtualcarrier.

However, in other examples, the virtual carrier PRACH 1303 is situatedoutside of the virtual carrier as shown for example in FIG. 13C. Thisleaves more room within the virtual uplink carrier for the transmissionof data by the virtual carrier terminals. The position of the virtualcarrier PRACH is signaled to the virtual carrier terminals as before butin order to transmit a random access request, the virtual carrierterminals re-tune their transmitter units to the virtual carrier PRACHfrequency because it is outside of the virtual carrier. The transmitterunits are then re-tuned to the virtual carrier frequency when uplinkresource elements have been allocated.

In some examples where the virtual carrier terminals are capable oftransmitting on a PRACH outside of the virtual carrier, the position ofthe host carrier PRACH can be signaled to the virtual carrier terminals.The virtual carrier terminals can then simply use the conventional hostcarrier PRACH resource to send random access requests. This approach isadvantageous as fewer PRACH resources have to be allocated.

However, if the base station is receiving random access requests fromboth conventional LTE terminals and virtual carrier terminals on thesame PRACH resource, it is necessary that the base station is providedwith a mechanism for distinguishing between random access requests fromconventional LTE terminals and random access requests from virtualcarrier terminals.

Therefore, in some examples a time division allocation is implemented atthe base station whereby, for example, over a first set of subframes thePRACH allocation is available to the virtual carrier terminals and overa second set of subframes the PRACH allocation is available toconventional LTE terminals. Accordingly, the base station can determinethat random access requests received during the first set of subframesoriginate from virtual carrier terminals and random access requestsreceived during the second set of subframes originate from conventionalLTE terminals.

In other examples, no mechanism is provided to prevent both virtualcarrier terminals and conventional LTE terminals transmitting randomaccess requests at the same time. However, the random access preamblesthat are conventionally used to transmit a random access request aredivided into two groups. The first group is used exclusively by virtualcarrier terminals and the second group is used exclusively byconventional LTE terminals. Accordingly, the base station can determinewhether a random request originated from a conventional LTE terminal ora virtual carrier terminal simply by ascertaining to what group therandom access preamble belongs.

Example Architecture

FIG. 14 provides a schematic diagram showing part of an adapted LTEmobile telecommunication system arranged in accordance with an exampleof the present invention. The system includes an adapted enhanced Node B(eNB) 1401 connected to a core network 1408 which communicates data to aplurality of conventional LTE terminals 1402 and reduced capabilityterminals 1403 within a coverage area (cell) 1404. Each of the reducedcapability terminals 1403 has a transceiver unit 1405 which includes areceiver unit capable of receiving data across a reduced bandwidth and atransmitter unit capable of transmitting data across a reduced bandwidthwhen compared with the capabilities of the transceiver units 1406included in the conventional LTE terminals 1402.

The adapted eNB 1401 is arranged to transmit downlink data using asubframe structure that includes a virtual carrier as described withreference to FIG. 5 and to receive uplink data using a subframestructure as described with reference to FIG. 13B or 13C. The reducedcapability terminals 1403 are thus able to receive and transmit datausing the uplink and downlink virtual carriers as described above.

As has been explained above, because the reduced complexity terminals1403 receive and transmit data across a reduced bandwidth on the uplinkand downlink virtual carriers, the complexity, power consumption andcost of the transceiver unit 1405 needed to receive and decode downlinkdata and to encode and transmit uplink data is reduced compared to thetransceiver unit 1406 provided in the conventional LTE terminals.

When receiving downlink data from the core network 1408 to betransmitted to one of the terminals within the cell 1404, the adaptedeNB 1401 is arranged to determine if the data is bound for aconventional LTE terminal 1402 or a reduced capability terminal 1403.This can be achieved using any suitable technique. For example, databound for a reduced capability terminal 1403 may include a virtualcarrier flag indicating that the data must be transmitted on thedownlink virtual carrier. If the adapted eNB 1401 detects that downlinkdata is to be transmitted to a reduced capability terminal 1403, anadapted scheduling unit 1409 included in the adapted eNB 1401 ensuresthat the downlink data is transmitted to the reduced capability terminalin question on the downlink virtual. In another example the network isarranged so that the virtual carrier is logically independent of theeNB. More particularly the virtual carrier may be arranged to appear tothe core network as a distinct cell so that it is not known to the corenetwork that the virtual carrier has any relationship with the hostcarrier. Packets are simply routed to/from the virtual carrier just asthey would be for a conventional cell.

In another example, packet inspection is performed at a suitable pointwithin the network to route traffic to or from the appropriate carrier(i.e. the host carrier or the virtual carrier).

In yet another example, data from the core network to the eNB iscommunicated on a specific logical connection for a specific terminaldevice. The eNB is provided with information indicating which logicalconnection is associated with which terminal device. Information is alsoprovided at the eNB indicating which terminal devices are virtualcarrier terminals and which are conventional LTE terminals. Thisinformation could be derived from the fact that a virtual carrierterminal would initially have connected using virtual carrier resources.In other examples virtual carrier terminals are arranged to indicatetheir capability to the eNB during the connection procedure. Accordinglythe eNB can map data from the core network to a specific terminal devicebased on whether the terminal device is a virtual carrier terminal or anLTE terminal.

When scheduling resources for the transmission of uplink data, theadapted eNB 1401 is arranged to determine if the terminal to bescheduled resources is a reduced capability terminal 1403 or aconventional LTE terminal 1402. In some examples this is achieved byanalysing the random access request transmitted on the PRACH using thetechniques to distinguish between a virtual carrier random accessrequest and a conventional random access request as described above. Inany case, when it has been determined at the adapted eNB 1401 that arandom access request has been made by a reduced capability terminal1402, the adapted scheduler 1409 is arranged to ensure that any grantsof uplink resource elements are within the virtual uplink carrier.

In some examples, the virtual carrier inserted within the host carriercan be used to provide a logically distinct “network within a network”.In other words data being transmitted via the virtual carrier can betreated as logically and physically distinct from the data transmittedby the host carrier network. The virtual carrier can therefore be usedto implement a so-called dedicated messaging network (DMN) which is“laid over” a conventional network and used to communicate messagingdata to DMN devices (i.e. virtual carrier terminals).

Further Example Applications of Virtual Carriers

Having set out the concepts of virtual carriers of the kind described inco-pending UK patent applications numbered GB 1101970.0 [2], GB1101981.7 [3], GB 1101966.8 [4], GB 1101983.3 [5], GB 1101853.8 [6], GB1101982.5 [7], GB 1101980.9 [8] and GB 1101972.6 [9], GB 1121767.6 [10]and GB 1121766.8 [11], some extensions of the virtual carrier concept inaccordance with embodiments of the invention are now described.

FIG. 15A is a schematic diagram representing how various regions in aLTE-type telecommunications network's time-frequency transmissionresource grid 1500 might be allocated for use to support a virtualcarrier such as described above. The extent of the resource grid 1500shown in FIG. 15A comprises 10 subframes 1512 (equivalent to one frameoverall) spaced along the horizontal time direction and spans abandwidth R₃₂₀ in frequency. Each subframe 1512 in FIG. 15A follows thesame general format as the subframe in FIG. 5 but is represented in amore simplified and schematic manner.

Thus, the transmission resource grid 1500 of FIG. 15A comprises hostcarrier PDCCH regions 1502, host carrier PDSCH regions 1506, virtualcarrier regions 1510 and reference symbol regions 1504. The virtualcarrier regions 1510 may comprise separate virtual carrier PDSCH regionsand virtual carrier PDCCH regions, such as schematically shown in FIG. 5by the separate regions identified by reference numerals 501 and 502.However, and as noted above, in other example implementations theprinciples of the virtual carrier operation might not mirror theseaspects of LTE-type networks. The reference symbol regions 1504 may beused solely for the host carrier, or these regions may also be receivedand used by terminals camped-on to the virtual carrier.

FIG. 15B is similar to and will be understood from FIG. 15A, but whereasFIG. 15A schematically represents regions of the time-frequencytransmission resource grid 1500 for both the host and virtual carriers,FIG. 15B schematically represents only regions associated with the hostcarrier (i.e. the host carrier PDCCH regions 1502, host carrier PDSCHregions 1506 and reference symbol regions 1504). In effect FIG. 15Brepresents what might be referred to as the host-carrier transmissionresource grid 1530. The regions of FIG. 15B shown without shading areassociated with the virtual carrier and do not “belong” to thehost-carrier transmission resource grid 1530.

FIG. 15C is also similar to and will be understood from FIG. 15A, butwhereas FIG. 15A schematically represents regions of the time-frequencytransmission resource grid 1500 for both the virtual and host carriers,FIG. 15C schematically represents only regions associated with thevirtual carrier (i.e. the virtual carrier regions 1510). FIG. 15C isthus the counter-part to FIG. 15B. In effect FIG. 15C represents whatmight be referred to as the virtual-carrier transmission resource grid1550. The regions of FIG. 15C shown without shading are associated withthe host carrier and do not “belong” to the virtual-carrier transmissionresource grid 1550.

The host-carrier transmission resource grid 1530 and virtual-carriertransmission resource grid 1550 complement one another in that one“fills” the spaces in the other so that when added together theycorrespond to the overall transmission resource grid 1510 of FIG. 15A.Thus to summarise some embodiments of the invention, communications aresupported using a plurality of Orthogonal Frequency DivisionMultiplexing, OFDM, sub-carriers spanning a first frequency bandwidth(e.g. R₃₂₀ in FIGS. 15A to 15C). User-plane data may be communicated onthe host carrier using a first group of the OFDM sub-carriersdistributed across the first frequency bandwidth (e.g. in regions 1506of FIG. 15B) and user-plane data may be communicated on the virtualcarrier using a second group of the OFDM sub-carriers distributed acrossa second frequency bandwidth, wherein the second frequency bandwidth issmaller than and within the first frequency bandwidth (e.g. withinregions 1510 of FIG. 15C). However, control-plane data for the hostcarrier (e.g. PDCCH) may be communicated using both groups of the OFDMsub-carriers (e.g. in regions 1502 of FIG. 15B).

FIG. 16 schematically shows a telecommunications system 1600 accordingto an embodiment of the invention. The telecommunications system 1600 inthis example is based broadly on an LTE-type architecture in which avirtual carrier, such as described above, is implemented. As such manyaspects of the operation of the telecommunications system 1600 are knownand understood and are not described here in detail in the interest ofbrevity. Operational aspects of the telecommunications system 1600 whichare not specifically described herein may be implemented in accordancewith any known techniques, for example according to the currentLTE-standards with appropriate modifications to support a virtualcarrier as has been previously proposed.

The telecommunications system 1600 comprises a core network part(evolved packet core) 1602 coupled to a radio network part. The radionetwork part comprises a base station (evolved-nodeB) 1604 coupled to aplurality of terminal devices. In this example, two terminal devices areshown, namely a first terminal device 1606 and a second terminal device1608. It will of course be appreciated that in practice the radionetwork part may comprise a plurality of base stations serving a largernumber of terminal devices across various communication cells. However,only a single base station and two terminal devices are shown in FIG. 16in the interests of simplicity.

As with a conventional mobile radio network, the terminal devices 1606,1608 are arranged to communicate data to and from the base station(transceiver station) 1604. The base station is in turn communicativelyconnected to a serving gateway, S-GW, (not shown) in the core networkpart which is arranged to perform routing and management of mobilecommunications services to the terminal devices in thetelecommunications system 1600 via the base station 1604. In order tomaintain mobility management and connectivity, the core network part1602 also includes a mobility management entity (not shown) whichmanages the enhanced packet service, EPS, connections with the terminaldevices 1606, 1608 operating in the communications system based onsubscriber information stored in a home subscriber server, HSS. Othernetwork components in the core network (also not shown for simplicity)include a policy charging and resource function, PCRF, and a packet datanetwork gateway, PDN-GW, which provides a connection from the corenetwork part 1602 to an external packet data network, for example theInternet. As noted above, the operation of the various elements of thecommunications system 1600 shown in FIG. 16 may be broadly conventionalapart from where modified to provide functionality in accordance withembodiments of the invention as discussed herein.

In this example, it is assumed the first terminal device 1606 is aconventional smart-phone type terminal device communicating with thebase station 1604 primarily using resources associated with the hostcarrier component of the radio interface (e.g. such as represented inFIG. 15B). This first terminal device 1604 comprises a transceiver unit1606 a for transmission and reception of wireless signals and acontroller unit 1606 b configured to control the smart phone 1606. Thecontroller unit 1606 b may comprise a processor unit which is suitablyconfigured/programmed to provide the desired functionality usingconventional programming/configuration techniques for equipment inwireless telecommunications systems. The transceiver unit 1606 a and thecontroller unit 1606 b are schematically shown in FIG. 16 as separateelements. However, it will be appreciated that the functionality ofthese units can be provided in various different ways, for example usinga single suitably programmed integrated circuit. As will be appreciatedthe smart phone 1606 will in general comprise various other elementsassociated with its operating functionality.

In this example, it is assumed the second terminal device 1608 is amachine-type communication (MTC) terminal device communicating with thebase station 1604 using resources associated with the virtual carriercomponent of the radio interface (e.g. such as represented in FIG. 15C).As discussed above, these types of device may be typically characterisedas semi-autonomous or autonomous wireless communication devicescommunicating small amounts of data. Examples include so-called smartmeters which, for example, may be located in a customer's house andperiodically transmit information back to a central MTC server datarelating to the customer's consumption of a utility such as gas, water,electricity and so on. MTC devices may in some respects be seen asdevices which can be supported by relatively low bandwidth communicationchannels having relatively low quality of service (QoS), for example interms of latency. It is assumed here the MTC terminal device 1608 inFIG. 16 is such a device.

As with the smart phone 1606, the MTC device 1608 comprises atransceiver unit 1608 a for transmission and reception of wirelesssignals and a controller unit 1608 b configured to control the MTCdevice 1608. The controller unit 1608B may comprise various sub-units,such as a monitoring unit, an identifying unit, a determining unit, andan initiating unit for providing functionality in accordance withembodiments of the invention as explained further below. These sub unitsmay be implemented as discrete hardware elements or as appropriatelyconfigured functions of the controller unit. Thus the controller unit1608 b may comprise a processor unit which is suitablyconfigured/programmed to provide the desired functionality describedherein using conventional programming/configuration techniques forequipment in wireless telecommunications systems. The transceiver unit1608 a and the controller unit 1608 b are schematically shown in FIG. 16as separate elements for ease of representation. However, it will beappreciated that the functionality of these units can be provided invarious different ways following established practices in the art, forexample using a single suitably programmed integrated circuit. It willbe appreciated the MTC device 1608 will in general comprise variousother elements associated with its operating functionality.

The base station 1604 comprises a transceiver unit 1604 a fortransmission and reception of wireless signals and a controller unit1604 b configured to control the base station 1604. The controller unit1606B may again comprise various sub-units, such as a scheduling unitand a selecting unit for providing functionality in accordance withembodiments of the invention as explained further below. These sub unitsmay be implemented as discrete hardware elements or as appropriatelyconfigured functions of the controller unit. Thus, the controller unit1604 b may comprise a processor unit which is suitablyconfigured/programmed to provide the desired functionality describedherein using conventional programming/configuration techniques forequipment in wireless telecommunications systems. The transceiver unit1604 a and the controller unit 1604 b are schematically shown in FIG. 16as separate elements for ease of representation. However, it will beappreciated that the functionality of these units can be provided invarious different ways following established practices in the art, forexample using a single suitably programmed integrated circuit. It willbe appreciated the base station 1604 will in general comprise variousother elements associated with its operating functionality.

Thus, the base station 1604 is configured to communicate data with thesmart phone 1606 over a first radio communication link 1610 associatedwith a host carrier of the wireless telecommunications system and tocommunicate data with the MTC device 1608 over a second radiocommunication link 1612 associated with a virtual carrier of thewireless application system.

It is assumed here the base station 1604 is configured to communicatewith the smart phone 1606 over the first radio communication link 1610in accordance with the established principles of LTE-basedcommunications supporting a host and virtual carrier, such as describedabove.

In accordance with previously proposed virtual carrier (VC) techniques,a VC terminal device (UE) in connected mode will search a control region(such as the control region 502 schematically represented in FIG. 5) toidentify possible allocations of downlink transmissions from the basestation which are scheduled for the VC terminal device (such as the MTCdevice 1608 presented in FIG. 16). This procedure on the virtual carriermay generally mirror the established techniques for LTE-basedcommunications on a conventional carrier. Thus, the control regionsearched by the virtual carrier terminal device to identify resourceallocations may correspond to what might be referred to as a VC-PDCCH.Searching VC-PDCCH for terminal device specific resource allocationsconsumes power at the terminal device. Furthermore, the consumed poweris in effect wasted if in fact the base station has not scheduled anyterminal device specific transmissions for the particular terminaldevice. The inventors have recognised that for MTC-type devices inparticular, the typically bursty and sporadic nature of MTC-typecommunications means it will often be the case that no terminal devicespecific data are scheduled for transmission to a given terminal device.Accordingly, the process of decoding VC-PDCCH in each subframe canrepresent a significant yet unnecessary drain on a terminal device'sresources.

MTC-type traffic can typically be reasonably well predicted in advanceby a network (and in particular by a scheduler in a base station).Furthermore even for unpredictable MTC-type traffic the traffic is inany case typically not tightly time-bounded (i.e. the data is delaytolerant). This means there is generally a significant degree offlexibility available to a base station in the scheduling of MTC-typedata/traffic. That is to say, a scheduling unit in a base station canplan to manage the transmission of data to MTC-type devices overrelatively long periods without significant impact on operationalperformance for the devices. For example, a base station may scheduleMTC-data relatively far in advance on a virtual carrier because thevirtual carrier is restricted in bandwidth resource and, particularly ifthere are a significant number of MTC devices needing broadlysimultaneous downlink resources (for example to broadcast a tariffchange to a plurality of smart meters), it may not be possible toschedule all relevant MTC terminal devices in one subframe. As explainedabove, any VC-terminal devices which are not scheduled in a givensubframe will in effect waste energy if they decode the virtual carriercontrol region for that subframe. Unnecessary consumption of energy canbe a particular concern for MTC terminal devices because they maytypically be designed to operate with relatively long intervals betweenbattery replacements or re-charges.

This scheduling profile and desire to reduce power consumption makesMTC-type devices well-suited to techniques which allow the devices toenter reduced-activity (sleep/suspension) modes, such as the known DRXand microsleep modes. However, as discussed above, there are drawbackswith these established sleep modes and so alternative techniques forcontrolling a terminal device to enter a reduced-activity mode in awireless telecommunications network are proposed in accordance withembodiments of the invention. The proposed techniques are well suited toMTC-type devices operating on a virtual carrier.

As explained above, an established aspect of LTE-type communications,including LTE-type telecommunications on a virtual carrier, is the useof reference symbols. These are interspersed throughout thetime/frequency resource grid of a downlink subframe to provide forchannel estimation and other purposes in accordance with knowntechniques. FIG. 17 schematically represents an arrangement of cellspecific reference symbols (CRS) in accordance with conventional LTEtechniques. FIG. 17 shows a region of an LTE-type downlink radio framestructure corresponding to 2 resource blocks (extending for 1 ms in time(one subframe/14 symbols) and 180 kHz in bandwidth (12 subcarriers)). Inaccordance with current LTE standards the extent of the resource gridrepresented in FIG. 17 will contain eight cell-specific referencesymbols. As is well established, the specific locations forcell-specific reference symbols transmitted by a base station aredetermined according to parameters such as the cell identity and antennaport. A terminal device connected to the base station is thus able tolocate and decode the reference symbols transmitted by the base stationfor channel estimation purposes.

In accordance with embodiments of the invention, a base station isconfigured to suppress transmission of one or more reference symbols inone or more radio subframes as a means of conveying to a terminal devicereceiving the reference symbols that it should enter a reduced activitystate/mode for a defined period of time. The defined period of time maybe established by the terminal device based on which reference symbol(s)have been suppressed. Thus, a scheduling unit in a base station mayestablish that a terminal device or group of terminal devices is notscheduled to receive any terminal-device specific data for a period oftime, and the base station may communicate this information to theterminal device(s) through suppression of an appropriate referencesymbol in accordance with a pre-established correspondence betweenreference symbols and potential period of time. Terminal device(s)monitoring the reference symbols transmitted by the base station maythus identify the suppression of the reference symbol (e.g. because itis not received), determine from the identity(ies)/location(s) of thereference symbol(s) which is(are) suppressed that the terminal device isnot scheduled to receive any (or a particular type of) terminal devicespecific data for the corresponding period of time, and enter a reducedactivity mode. For example, the terminal device may enter a reducedactivity mode in which the terminal device does not decode controlinformation regarding resource allocations for the determined period oftime.

Communicating information regarding periods of time during whichterminal devices may suspend certain (or all) decoding tasks throughselective puncturing (suppression) of reference symbol transmissions bya base station in this way allows such information to be carriedimplicitly at the physical layer. Accordingly, this signalling toindicate that a reduced-activity mode may be entered for a given periodof time can be sent quickly and without using resource-hungry RRCsignalling.

Where cell specific reference symbols are suppressed there willgenerally be a plurality of terminal devices receiving the cell specificreference symbols and these plurality of terminal devices may allrespond in the same way (i.e. so multiple terminal devices can becontrolled to enter a reduced activity state for the same period). Forexample, it may be helpful to instruct all terminal devices on a virtualcarrier to enter a reduced activity mode because the virtual carrier isto be suspended for a period of time to accommodate more data fordevices not using the virtual carrier. In other examples, and asexplained further below, different terminal devices may be configured torespond to different suppressed reference symbols in different ways,thereby allowing the base station to selectively control subsets ofterminal devices to enter the reduced activity state. For example, onesubset of terminal devices might comprise smart meters associated withcompany A, while another subset of terminal devices might comprise smartmeters associated with company B, and activation of a reduced activitystate for each company's devices might be separately controlled.However, embodiments of the invention are not restricted to suppressionof cell specific reference symbols. For example, in accordance with someembodiments the base station may be configured to suppress UE specificreference symbols (e.g. DM-RS) that a particular terminal device (UE) isotherwise expecting, thereby providing a scheme for communicating to aspecific terminal device that it may enter the reduced activity statefor a particular period of time. Similarly, other embodiments of theinvention may be based around suppression of demodulation referencesymbols and/or channel state information reference symbols and/orpositioning reference symbols.

Embodiments of the invention may thus allow for reduced powerconsumption in terminal devices by reducing the signal processing to beperformed in subframes during which the terminal device is instructed tosuspend decoding of certain transmissions it would otherwise decode, forexample VC-PDCCH, and also reduced power consumption by terminal devicetransceivers during these subframes.

By comparison to the known technique of DRX, embodiments of theinvention can also reduce the latency associated with a VC-terminaldevice resuming connected mode access to the network. Thus embodimentsof the invention may combine an ability to achieve power savings fromdisabling some parts of receive processing, as in DRX, but without theincreased latency that DRX brings. Furthermore, whereas DRX iscontrolled at RRC, whereas embodiments of the invention are controlledat the physical layer (possibly with some initial higher-layersetup/configuration signalling as discussed further below), a suspendeddecoding/reduced activity mode may be activated in accordance withembodiments of the invention more quickly than for DRX and with lessoverhead resource usage.

By comparison to the known technique of PDCCH microsleep, embodiments ofthe invention may also provide signal processing power savings becausethe terminal devices need not search and decode control information(e.g. PDCCH) for subframes in which decoding suspension/reduced activitymode is indicated. What is more, with the microsleep technique aterminal device must “awake” to decode PDCCH in each and every subframe,thereby restricting the length of time for which the terminal device cansave power. In accordance with embodiments of the invention, a period oftime greater than an individual subframe can readily be set as theduration of a reduced activity mode.

A consequence of a suppressed reference symbol approach in accordancewith embodiments of the invention is that in some cases a terminaldevice which fails to correctly receive a reference symbol that is infact transmitted by the base station, for example because ofinterference, may erroneously enter a reduced activity mode. If a basestation then transmits data to the terminal device the terminal devicewill fail to receive it. However, the established retransmissionprotocols of LTE, for example based around ACK/NACK signalling, can beused to automatically alert the base station to the need to retransmitthe data. Also, and as explained further below, in accordance with someembodiments of the invention there are techniques that can be adopted toreduce the risk of this happening.

Thus, in accordance with one embodiment of the invention, it is assumedthat a downlink scheduler (scheduling unit) in a base station supportinga virtual carrier has determined that it will not schedule a certaintype of terminal-specific downlink data for a VC terminal device for aparticular number of subframes, although it wishes to resumetransmissions (or at least the possibility of transmissions) to theterminal device after that number of subframes. It will be appreciatedthat a scheduler may establish this in accordance with any knowntechniques for scheduling transmissions in a wireless telecommunicationssystem.

To convey this information to the terminal device (thereby allowing theterminal device to enter the reduced activity mode), the base station isconfigured to signal to the terminal device that the terminal deviceneed make no attempt to locate or decode VC-PDCCH (or other decodingaccording to the implementation at hand) for the relevant number ofsubframes. This may be signalled, for example, in the final subframebefore the suspension of transmission to the terminal device is tobegin. As explained above, in accordance with embodiments of theinvention the base station may convey the information through signallingat the physical layer by suppressing transmission of a particularreference signal (RS) in a particular resource element (RE) (or acombination of multiple reference symbols) where the terminal device isotherwise expecting it to occur in accordance with an establishedpattern of reference symbols for the wireless communication system.

A terminal device monitoring the reference symbols will identify thatthe suppressed reference symbol is not received, and may be configuredto determine from this that it may enter a reduced activity state inwhich it suspends decoding of VC-PDCCH for the relevant period of time.As mentioned above, this process may be based on any of cell specificreference symbols (CRS), demodulation reference symbols (DM-RS) (UEspecific reference symbols), control state indicator reference symbols(CSI-RS), or positioning reference symbols (P-RS) which are beingtransmitted to a relevant terminal device in a relevant subframe.

In a simple implementation a terminal device may be configured to entera reduced activity state for a fixed number of subframes, for example 10subframes, whenever it identifies that an expected reference symbol isnot received. However, in general it may be preferable to provide moreflexibility in the information communicated to a terminal device(example different durations of reduced activity). This can be done, forexample, by establishing a correspondence between particular referencesymbols and particular periods of time such that suppression ofdifferent reference symbols conveys an indication that a terminal devicemay enter a reduced activity mode for different periods of time. Such acorrespondence can be established in a standard of the wirelesscommunication system, or may be established by a base station andcommunicated to terminal devices using higher layer signalling, forexample during a camp-on procedure when a terminal device first connectsto the base station. There are many different forms of correspondencethat can be established. For example, in some cases it may beestablished that a particular suppressed reference symbol (orcombination of reference symbols) in any resource block corresponds witha particular period of time (i.e. the choice of punctured referencesymbol with a resource block/subframe conveys information). In othercases it may be established that suppression of any reference symbol ina particular resource block corresponds with a particular period of time(i.e. the choice of resource block/subframe containing a puncturedreference symbol conveys the information).

FIG. 18 schematically represents a correspondence between referencesymbols and potential periods of time for which decoding may besuspended by a terminal device in accordance with an embodiment of theinvention. FIG. 18 is similar to, and will be understood from, FIG. 17.However, in accordance with an embodiment of the invention, the basestation and the terminal device(s) operating in a wirelesstelecommunications system configured to implement an embodiment of thepresent invention are both aware of a pre-established correspondencebetween individual reference symbols in each resource block pair of asubframe and a potential period of time for which terminal devicedecoding may be reduced (i.e. at least partially suspended). An examplecorrespondence is schematically represented in the figure. This allows abase station to convey an indication that a terminal device may enter areduced activity state for any number of subframes between one and eightdepending on which reference symbol is suppressed/punctured. Longerperiods of time can be indicated by suppressing multiple referencesymbols. For example, a suspension of 14 subframes might be indicated bysuppressing the reference symbol corresponding with a suspension of 6subframes and the reference symbol corresponding with a suspension of 8subframes. In the example represented in FIG. 18 a reference symbolestablished as corresponding to a suspension of 5 subframes isschematically represented as being punctured (not transmitted). Thus,the base station does not transmit this reference symbol. A terminaldevice implementing an embodiment of the invention recognises thisreference symbol has been suppressed, and may accordingly initiate areduced activity mode starting from the following subframe for a periodof five subframes. Assuming only a single reference symbol is to bepunctured (to minimise the impact on devices using the reference symbolsfor conventional channel estimation), the correspondence represented inFIG. 18 allows for 8 possible durations of decoding suspension to beindicated (i.e. one per CRS location). Thus, in the narrowest systembandwidth currently supported in LTE (1.4 MHz bandwidth—equivalent to awidth of 72 subcarriers/six resource blocks) there are 48 possibledurations which could be indicated with each requiring only a singlereference symbol to be suppressed. Thus, there can be significantflexibility in how long terminal devices can be controlled to enter areduced activity state with relatively low impact on other devices usingthe reference symbols for channel estimation. Conventional channelestimation techniques allow for the possibility of “lost” referencesymbols, for example because of interference, and so a conventionalterminal device not implementing an embodiment of the invention will beable to continue operating as normal even with the deliberate selectivesuppression of reference symbol transmissions.

In the case that more than one reference symbol may besuppressed/punctured within the 1.4 MHz bandwidth of a single subframe,a correspondingly expanded number of options regarding the potentialsuspension durations that may be communicated (or other elements ofinformation as described further below) can be conveyed according to howmany potential puncturing patterns are defined. For example, if anycombination of two reference symbols may be suppressed to indicatedifferent potential decoding suspension durations, then in the case ofCRS such as illustrated above, there are 48C2=(48*47/2)=1128 possibledifferent durations which can be indicated in the narrowest systembandwidth of 6 resource blocks width.

As noted above, suppression of reference symbols may degrade theperformance of the channel estimation process for conventional “legacy”devices relying on the reference symbols for channel estimation.However, this can be mitigated by ensuring only a relatively smallfraction of reference symbols are suppressed (for example less than 5%).Furthermore, more advanced terminal devices, even if not implementing anembodiment of the invention, may be configured to receive signallingfrom a base station indicating which reference symbols may be suppressedto help them maximise the channel estimation process.

In a case where the base station may not be aware as to whether aparticular terminal device will be decoding a particular referencesymbol it wishes to puncture, such as when DM-RS and CRS are bothtransmitted in transmission mode 7, one approach may be to configurepuncturing of reference symbols on all relevant reference symbols, andinclude in a higher-layer configuration step, for example during a campon procedure, an indication to respond to only one of them.

In the above example it is assumed suppression of a reference symbolcorresponds with not transmitting a reference symbol. However, in otherexamples suppressing a reference symbol may involve simply transmittingthe reference symbol at a different power as compared to other referencesymbols, for example, a power which is lower than an average power forthe reference symbols by more than a threshold amount. In a moreadvanced implementation a base station may identify that a terminaldevice which is to be controlled into a reduced activity state mode islocated a long way from the base station such that the base station maytransmit a reference symbol with sufficiently low power that it cannotbe received by the remote terminal device, but can still be received bycloser terminal devices, albeit with reduced power. Broadly similarprinciples can be applied using beam forming to control where in a cella reference symbol will appear to be suppressed as compared to otherlocations.

In accordance with some embodiments of the invention reference symbolswhich may be punctured to convey suspension information may betransmitted on more than one antenna port of the base station. In thiscase, the puncturing might not be applied to all antenna ports.Furthermore, the different possible combinations of antenna ports onwhich the puncturing of particular resource elements may be applied canbe used to indicate more possible options for configuring terminaldevices into respective reduced activity states. For example, forreference symbols transmitted using antenna ports ‘a’ and ‘b’,puncturing on only port ‘a’, on only port ‘b’, or on both port ‘a’ andport ‘b’ serve as three more states for conveying information.

One example usage of this approach could be in the case of a basestation and terminal device supporting 4-port transmission of, e.g. CRS.On ports 2 and 3, the reference symbol density is half that of ports 0and 1, taking account of the fact that if the system is using high-orderspatial-multiplexing provided by four ports, the radio channel isinherently likely to be one having high SINR and low mobility, and thuscan be well-estimated with reduced reference symbol overhead. However,if the system is using fewer than four ports for PDSCH transmissions,then reference symbol puncturing in accordance with embodiments of theinvention could be applied to the unused port(s) to reduce degradationof channel estimation on the ports for which PDSCH transmission isexpected to occur.

In accordance with another embodiment of the invention the selectedreference symbol puncturing may also be used to indicate furtherinformation, for example a delay until the suspension begins. Forexample, in a simple case some reference symbols in a subframe may beused to indicate different potential periods of time for which aterminal device may enter a reduced activity state, whilst otherreference symbols in the subframe may be used to indicate a delay (e.g.in terms of a number of subframes following the current subframe), afterwhich the period of reduced activity is to begin. Thus, the base stationmay suppress one reference symbol to indicate a duration for the reducedactivity state and another reference symbol to indicate a start time forentering the reduced activity state, or one reference symbol mayindicate both the delay after which the reduced activity is to begin andits duration, for example based on a pre-established correspondencebetween different reference symbols that might be suppressed anddifferent combinations of these parameters.

In accordance with some embodiments the selective suppression ofspecific reference symbols may be used to indicate patterns of futuresubframes for which the terminal device may enter a reduced activitystate rather than simply a single continuous period. For example, asubset of reference symbols may be associated with different durationsof reduced activity state, whilst other reference symbols may beassociated with different patterns for application of the reducedactivity state. For example, suppression of a particular referencesymbol may be associated with an indication that a reduced activitystate should be cyclically entered. Thus, the base station may suppressone reference symbol to indicate a particular duration for a reducedactivity state, such as described above, and also suppress in the same,or a related, subframe, a reference symbol to indicate this state may berepeatedly entered. A terminal device may respond by entering thereduced activity state for the appropriate duration, and then exit thereduced activity state for the same duration, and then re-enter thereduced activity state for the duration, and so forth. The terminaldevice may be configured to continue to do this until the base stationsuppresses another reference symbol associated with deactivating thismode of operation (it will be appreciated that where the presentdescription refers to suppression of particular reference symbols toindicate corresponding information, this should be interpreted as alsoreferring to suppressing particular combination of reference symbols toindicate corresponding information). Different patterns for entering andexiting the reduced activity state can be associated with differentreference symbols in association with a pre-established scheme.

Thus in accordance with some embodiments of the invention the selectivesuppression of specific reference symbols (or combinations of referencesymbols) can be used to also convey further information from the basestation to the terminal device. That is to say, the selectivesuppression of at least one reference symbol may be used to conveyinformation generally from the base station to the terminal device(s),and not just information regarding a period of time during which theterminal device may enter a reduced activity state. For example, acombination of a delay duration indicated by suppression of a referencesymbol in a subframe, and the subframe number in which it is indicated,could be used to convey information regarding the width (in terms ofsymbols) of the PDCCH control region of the host carrier in which aterminal device is to resume decoding the virtual carrier PDCCH. Thiscould therefore simplify the process of the terminal device determiningthe location of the VC-PDCCH in the first subframe after suspension ofdecoding (for example, where the virtual carrier PDCCH immediatelyfollows the host carrier PDCCH, as opposed to being at the end of thesubframe as schematically represented in FIG. 5). However, this approachwould potentially restrict the scheduling flexibility of the basestation in the first subframe after suspension since a promise hasalready been made regarding the width of the control region before itcan be guaranteed that it is correct when the time comes to transmit it.There are various approaches for handling this where such a feature isimplemented, such as (a) simply tolerating the scheduling inefficiency;or (b) if the inefficiency is judged too great, simply breaking thepromise made to the VC-terminal device. In the latter case, the terminaldevice may fail to properly decode the virtual carrier PDCCH in thefirst subframe after expiry of the period of the reduced activity on itsfirst attempt. However, the terminal device can resort to blind decodingover the remaining possible PDCCH control-region widths until it issuccessful.

The above-described embodiments have focused on how a terminal devicemight be configured to suspend decoding of a virtual carrier controlregion associated with the transmission of terminal device specificinformation. However, in accordance with different embodiments of theinvention there may be different degrees of suspension in the reducedactivity state. For example, as well as suspending the decoding ofVC-PDCCH, and hence also the virtual carrier PDSCH, in other examples aterminal device might suspend decoding of an entire subframe, forexample including all reference symbols, PBCH, and synchronisationsignalling, or any subset of these. This may in some circumstances beless desirable since a terminal device might, for example, losesynchronisation with the cell or be unaware of changes to the MIB, butsome further power saving at the device would be achieved which could bedesirable in some MTC applications.

As noted above, suppression of reference symbols in accordance withembodiments of the invention could potentially reduce the quality ofchannel estimation or feedback for terminal devices which are operatingin the wireless telecommunications network and using the referencesymbols for channel estimation. In accordance with some embodimentsnon-suppressed reference symbols could be transmitted with higher power(power-boosted) to increase the reliability with which terminal devicescan detect these reference symbols. The transmission powers of thereference symbols could be signalled to terminal devices using existingmeans.

As also noted above, it may in principle happen that, although the basestation does transmit a reference symbol in a given resource element,the transmission may not be properly received by the terminal device.The terminal device may thus interpret this incorrectly as an indicationto suspend VC-PDCCH decoding. Thus, in accordance with some embodiments,there may be a requirement for multiple reference symbols to beidentified as suppressed before entering a reduced activity state. Forexample, in one example there may be a requirement for a particularreference symbol puncturing pattern to be detected in two or moresubframes for a terminal device to interpret this as a positiveindication that it may enter a reduced activity state for a givenperiod.

It will be appreciated that there are an enormous number of differentways in which specific information may be conveyed throughsuppression/puncturing of reference symbols in accordance withembodiments of the invention. For example, the correspondence mappingsrepresented in FIG. 18 provides a simple scheme for communicating apotential period of inactivity of between one and eight subframes withinthe portion of the downlink subframe represented in the figure. As hasbeen mentioned, for a virtual carrier bandwidth of 1.4 MHz, there wouldbe 48 possible puncturing locations for CRS in one subframe (TTI).Following the simple approach of FIG. 18, this would allow communicationof potential suspension periods of between 1 and 48 subframes (ms) withsingle resource element puncturing. However, it will be appreciated thatin any given implementation there is a large degree of freedom inmapping specific reference symbols to specific potential periods forwhich a terminal device may enter a reduced activity state in accordancewith embodiments of the invention. The correspondence between particularreference symbols and potential periods of time for the terminal devicemade a reduced activity state may be established in a look-up table.Such a look-up table may be defined in accordance with a standard of thewireless telecommunications system, or may be established by a basestation in accordance with current conditions, and then communicated toterminal devices using higher layer signalling, for example during acamp-on procedure. In effect, the relationship between individualreference symbols and corresponding period of time in a givenimplementation may be entirely arbitrary. In other examples there may bea formulaic relationship between reference symbols selected forsuppression and corresponding periods of suspension.

For example, in one look-up table based example it may be decided thatproviding for values of between 1 ms and 48 ms in 1 ms increments isappropriate. However, in another example it may be considered preferableto allow for longer periods of suspension. Thus, perhaps 30 referencesymbols will be associated with periods of time from 1 to 30 ins, andthe remaining 18 reference symbols may be associated with significantlylonger periods of time.

In some cases it may be desirable for a base station to be able toselectively indicate to different identities of terminal device that thedevice may enter a reduced activity state. In accordance with someembodiments of the invention, such an indication may also be providedbased on the selective suppression of at least one reference symbol.Thus, in accordance with some embodiments, a reference symbol puncturinglocation may be used for not only indicating a suspension duration, butalso for indicating a terminal device identification. This may beachieved, for example, by in effect allocating certain reference symbolsto certain connected terminal devices. These may be allocated during acamp-on procedure for each terminal device or through other explicitsignalling between the base station and the terminal device. Forexample, referring to the above case where there are 48 potentialreference symbols for suppression in a given subframe of a 1.4 MHzbandwidth virtual carrier, if there are eight connected terminal devicesimplementing an embodiment of the invention, each terminal device may beassociated with a group of six reference symbols, thereby allowing thebase station to individually control each of the eight terminal devicesto enter a reduced activity state for six potential periods (one perreference symbol allocated to the terminal device). A drawback of thisapproach is a reduction in the number of potential durations that may besignalled to a terminal device with increasing number of terminaldevices which are to be individually addressed.

As well as using this approach to address individual terminal devices, asimilar scheme could be used to address groups of terminal devices. Forexample, terminal devices might be associated with a number of groups ofterminal devices (e.g. smart meters belonging to different companies).Terminal devices belonging to one company may all be associated with afirst subset of reference symbols whilst terminal devices belonging to adifferent company may all be associated with a different subset ofreference symbols. Thus, the base station can control the terminaldevices of each company by appropriate puncturing of the referencesymbols associated with the terminal devices of that company. Again, theassociation between reference symbols and terminal device identities canbe established through prior higher layer signalling (i.e. higher than aphysical layer), for example during a camp-on procedure, or in principlecan be standardised in the wireless telecommunications system.

When addressing groups of terminal devices as exemplified above, groupsof terminal devices whose durations of reduced activity states aremultiples of one another, such as when the durations are an even numberof subframes or for example when they are durations in subframes ofpowers of two, will potentially all resume ordinary processing in thesame subframe. This could make it difficult for the scheduler in thebase station to allocate resources to all the awakening terminaldevices. Therefore, different groups of terminal devices may beassociated with durations of the reduced activity state which are notmultiples of one another, such as durations of prime numbers ofmilliseconds, or odd numbers of milliseconds.

As noted above, a mapping between a reference symbol puncturing patternand a terminal device identifier may be established by the base stationand communicated to a terminal device through explicit signalling, suchas RRC signalling. An example of this approach is schematicallyrepresented in FIG. 19 which shows a ladder-type diagram for somesignalling steps between the base station 1604 and the terminal device1608 of FIG. 16 in accordance with an embodiment of the invention. In afirst step, for example in association with a camp-on procedure, thebase station 1604 configures the terminal device via RRC signalling withthe location(s) of potentially punctured reference symbols which areallocated to that terminal device and which that terminal device shouldsubsequently monitor for possible suppression (in a grouped-devicescenario, multiple terminal devices may be configured to monitor thesame reference symbols). In response to receiving the configurationinformation from the base station, the terminal device 1608 responds tothe base station to indicate when the configuration is complete. Theterminal device 1608 may then proceed to monitor the relevant referencesignals to seek to identify any suppressions. As represented in FIG. 19,the base station sends a punctured reference signal for the UE toindicate the UE may enter a period of reduced activity in accordancewith the principles described above. The terminal device detects thepunctured reference symbol and enters the reduced activity state (sleepmode) for the subframes associated with the punctured reference symbolreceived from the base station. Other terminal devices may be configuredto respond to the suppression of other reference symbols, therebyallowing the base station to selectively control activation of thereduced activity mode at different terminal devices through suppressionof the relevant reference symbols.

In other embodiments of the invention, different reference symbols whichmay be suppressed may be associated with different terminal devicesthrough implicit signalling based on an existing identifier for theterminal device. In LTE there is a range of different identifiers forterminal devices, such as a device's IMSI (international mobilesubscriber identity) or C-RNTI (cell-radio network temporaryidentifier). These identifiers are generally too large to be directlyaddressed through different, combinations of suppressed referencesymbols, although in principle this could be done with a sufficientlylarge set of reference symbols which may be punctured, for example a setspanning several subframes. However, terminal devices can be configuredto derive a further identifier based on an existing identifier. Forexample, a terminal device might be configured to establish a furtheridentifier based on a modulo division of one of their existingidentifiers. For example, a terminal device may establish a furtheridentifier as N=I modulo P, where N is the further identifier, I is theexisting identifier, and P is a predefined number corresponding to thenumber of different further identifiers that can be supported. Thus, aterminal device may establish it is associated with a given furtheridentifier and, by reference to an established standard of the wirelesstelecommunications system, may determine that this further identifierassociates the terminal device identity with a given subset of thereference symbols which may be suppressed. Thus, a number of differentterminal devices connected to a base station can implicitly derivedifferent identifiers that allow the base station to selectivelyactivate the reduced activity mode based on the selective suppression ofreference symbols. It will be appreciated that there will be a chancethat multiple terminal devices will derive the same further identifier(and indeed this will be a certainty when there are more terminaldevices then P connected to the base station). However, the base stationcan simply schedule sleep periods for terminal devices sharing the samefurther identifier at the same time.

Because embodiments of the invention can provide for relatively longduration suspensions/reduced activity mode (for example 10 ms or more),it is possible that channel conditions may be significantly different atexpiry of reduced activity period as compared to the beginning of thereduced activity period. Accordingly, it can be advantageous in someexamples for a terminal device to be configured to automaticallydetermine and transmit a channel quality indicator (CQI) to the basestation on exit from a period of reduced activity. In principle, a basestation can configure a terminal device to force a CQI report throughRRC signalling, but it can in some cases nonetheless be advantageous forthe terminal device to automatically send a CQI report using itsexisting periodic CQI configuration to avoid delays and additionalsignalling overhead.

Furthermore, it is possible that a terminal device which has beencontrolled to enter a reduced activity mode for a period of time mayneed to transmit uplink data to the base station. Thus, a terminaldevice may be configured to exit the reduced activity mode andcommunicate this to the base station through conventional random accesschannel (RACH) or scheduling request (SR) signalling. In the case of theterminal device transmitting SR signalling, the terminal device couldresume decoding VC-PDCCH in the next subframe to allow for thepossibility of an immediate uplink grant being sent from the basestation. In the case of the terminal device using the RACH procedure ona PRACH associated with the virtual carrier, the terminal device mayremain in a reduced activity state, for example with reduced decoding ofVC-PDCCH, until the start of the appropriate random access response(RAR) window, which may be several subframes later.

FIG. 20 is a flow diagram schematically representing processing in aterminal device in accordance with an embodiment of the invention. In afirst step S1 the terminal device establishes a mapping betweenreference symbols which may be suppressed and potential decodingsuspension periods. This may be achieved in accordance with any of theabove-identified techniques, such as through a standardisedmapping/look-up table or through signalling from a base station, such asRRC signalling, or in SIB (system information block)/MIB (masterinformation block) signalling. For example, wireless telecommunicationsstandards associated with the system may specify a plurality ofdifferent potential look-up tables mapping different combinations of atleast one resource element to different potential suspension durations,and SIB or MIB signalling may be used to indicate to a terminal devicewhich table to use.

In a second step S2 the terminal monitors each subframe to determine theresource elements (RE) associated with reference symbols that have beensuppressed (if any).

In a third step S3 the terminal device determines whether the resourceelements associated with reference symbols that have been suppressedmatch any of the mappings which are relevant for the terminal deviceestablished in step S1. If it is determined there is no match,processing follows the branch marked “NO” back to step S2 for iterativethe processing of the next subframe (subframe n+1) when it is received.If it is determined there is a match, the terminal device establishes asubframe p for starting decoding suspension and a subframe q forterminating decoding suspension based on the arrangement of at least onereference symbols for which transmission is determined to be suppressed.This can be done in accordance with any of the above-describedtechniques. For example, subframe p may be simply the next subframe(subframe n+1), and subframe q may be established by adding a period oftime associated with the particular matched pattern of suppressedreference symbols to the time of subframe p. Processing then follows tostep S4.

In step S4 the terminal device continues to process subframes as normaluntil subframe n=p−1 (in an example where p is simply the next subframeafter the match identified in step S3, there will be no subframesprocessed in step S4 as the condition n=p−1 is immediately met).

In step S5 the terminal device operates in a reduced-activity mode inwhich at least part of the normal mode receive/decoding processing isdisabled starting from subframe p.

In step S6 the terminal device determines that subframe q has beenreached, e.g. based on an internal timer or based on subframe-basedsignalling that continues to be received in the reduced activity mode.This concludes the process of the base station controlling the reducedactivity state of the terminal device for the defined period of time andprocessing returns to step S2.

It will be appreciated that various modifications can be made to theembodiments described above without departing from the scope of thepresent invention as defined in the appended claims. In particularalthough embodiments of the invention have been described with referenceto an LTE mobile radio network, it will be appreciated that the presentinvention can be applied to other forms of network such as GSM, 3G/UMTS,CDMA2000, etc. The term MTC terminal as used herein can be replaced withuser equipment (UE), mobile communications device, terminal device etc.Furthermore, although the term base station has been usedinterchangeably with eNodeB it should be understood that there is nodifference in functionality between these network entities.

Thus, there has been described a wireless telecommunications systemcomprising a base station and a terminal device and employing a radiointerface having a downlink radio frame structure comprising radiosubframes including an arrangement of reference symbols for channelestimation. The base station is configured to determine a period of timefor which certain terminal device specific data are not scheduled fortransmission to the terminal device and to communicate this informationto the terminal device through selective suppression of at least onereference symbol. Different reference symbol(s) may be suppressed toindicate different periods of time. The terminal device is configured tomonitor the reference symbols transmitted by a base station to identifywhere reference symbols are suppressed. The terminal device may thusdetermine from which reference symbols are suppressed a period of timefor which the terminal device is not expected to receive certain typesof data and enter a reduced activity mode for that period to conserveprocessing and power resources. Puncturing reference symbols in this wayprovides for fast physical-layer signalling of periods during which theterminal device may conserve resources by decoding fewer transmissionsthan it might otherwise need to do.

Embodiments may comprise a method of operating a base station to conveyto a terminal device information regarding a period of time for which atype of terminal device specific data is not scheduled for transmissionto the terminal device in a wireless telecommunications system employinga radio interface having a downlink transmission structure including anarrangement of reference symbols the reference symbols comprisingpredefined signals in predefined time and frequency resources, themethod comprising: determining a period of time for which terminaldevice specific data are not scheduled for transmission to a terminaldevice; selecting at least one reference symbol in at least one time andfrequency resource in dependence on the determined period of time; andsuppressing transmission of the at least one reference symbol in the atleast one time and frequency resource to indicate to the terminal devicethe period of time for which the type of terminal device specific dataare not scheduled for transmission to the terminal device.

Further particular and preferred aspects of the present invention areset out in the accompanying independent and dependent claims. It will beappreciated that features of the dependent claims may be combined withfeatures of the independent claims in combinations other than thoseexplicitly set out in the claims.

REFERENCES

-   [1] ETSI TS 122 368 V10.530 (2011-07)/3GPP TS 22.368 version 10.5.0    Release 10)-   [2] UK patent application GB 1101970.0-   [3] UK patent application GB 1101981.7-   [4] UK patent application GB 1101966.8-   [5] UK patent application GB 1101983.3-   [6] UK patent application GB 1101853.8-   [7] UK patent application GB 1101982.5-   [8] UK patent application GB 1101980.9-   [9] UK patent application GB 1101972.6-   [10] UK patent application GB 1121767.6-   [11] UK patent application GB 1121766.8

The invention claimed is:
 1. A method of operating a terminal device ina wireless telecommunications system employing a radio interfaceincluding an arrangement of downlink reference symbols, the methodcomprising: monitoring reference symbols transmitted by a base station;identifying that transmission by the base station of at least onereference symbol from the arrangement of downlink reference symbols issuppressed; determining a period of time for which to enter a reducedactivity mode based on the identified at least one reference symbol forwhich transmission is suppressed; and initiating the reduced activitymode for the determined period of time, wherein the radio interfacecomprises a plurality of Orthogonal Frequency Division Multiplexing(OFDM) sub-carriers spanning a system frequency bandwidth, the radiointerface supports a first carrier for communicating with a first classof terminal device using a first group of the OFDM sub-carriersdistributed across the system frequency bandwidth, and a second carrierfor communicating with a second class of terminal device on a secondgroup of the OFDM sub-carriers distributed across a restricted frequencybandwidth that is narrower than and within the system frequencybandwidth, and the terminal device is a terminal device of the secondclass operating on the second carrier.
 2. The method of claim 1, whereinthe determined period of time for which to enter the reduced activitymode is based on an association between different ones of the referencesymbols and different potential periods of time for entering the reducedactivity mode.
 3. The method of claim 2, wherein the association betweendifferent ones of the reference symbols and different potential periodsof time for entering the reduced activity mode is pre-defined for thewireless telecommunications system.
 4. The method of claim 2, whereinthe association between different ones of the reference symbols anddifferent potential periods of time for entering the reduced activitymode is communicated to the terminal device from the base station.
 5. Amethod of operating a terminal device in a wireless telecommunicationssystem employing a radio interface including an arrangement of downlinkreference symbols, the method comprising: monitoring reference symbolstransmitted by a base station, identifying that transmission by the basestation of at least one reference symbol from the arrangement ofdownlink reference symbols is suppressed; determining a period of timefor which to enter a reduced activity mode based on the identified atleast one reference symbol for which transmission is suppressed; andinitiating the reduced activity mode for the determined period of time,wherein the determined period of time for which to enter the reducedactivity mode is based on an association between different ones of thereference symbols and different potential periods of time for enteringthe reduced activity mode, and the association between different ones ofthe reference symbols and different potential periods of time forentering the reduced activity mode is defined in a look-up table.
 6. Amethod of operating a terminal device in a wireless telecommunicationssystem employing a radio interface including an arrangement of downlinkreference symbols, the method comprising: monitoring reference symbolstransmitted by a base station; identifying that transmission by the basestation of at least one reference symbol from the arrangement ofdownlink reference symbols is suppressed; determining a period of timefor which to enter a reduced activity mode based on the identified atleast one reference symbol for which transmission is suppressed; andinitiating the reduced activity mode for the determined period of time,wherein the at least one reference symbol for which transmission issuppressed comprises more than one reference symbol and the period oftime for entering the reduced activity mode is determined according to amapping between different combinations of reference symbols and aplurality of potential periods of time for entering the reduced activitymode.
 7. A method of operating a terminal device in a wirelesstelecommunications system employing a radio interface including anarrangement of downlink reference symbols, the method comprising:monitoring reference symbols transmitted by a base station; identifyingthat transmission by the base station of at least one reference symbolfrom the arrangement of downlink reference symbols is suppressed;determining a period of time for which to enter a reduced activity modebased on the identified at least one reference symbol for whichtransmission is suppressed; and initiating the reduced activity mode forthe determined period of time, wherein the reference symbols arereceived on transmissions from multiple antenna ports of the basestation and the determined period of time for entering the reducedactivity mode is based on which antenna port is associated with the atleast one reference symbol for which transmission is suppressed.
 8. Themethod of claim 1, wherein the reference symbols comprise at least oneof cell-specific reference symbols, terminal device specific referencesymbols, demodulation reference symbols, channel state informationreference symbols, or positioning reference symbols.
 9. The method ofclaim 1, wherein a start time for the period of time for entering thereduced activity mode relative to a time at which the transmission ofthe at least one reference symbol is suppressed is also based on the atleast one reference symbol for which transmission is suppressed.
 10. Themethod of claim 1, further comprising determining at least one furtherperiod of time for which to enter a reduced activity mode based on theidentified at least one reference symbol for which transmission issuppressed.
 11. The method of claim 10, wherein the determined period oftime and at least one further period of time follow a pattern definedaccording to the at least one reference symbol for which transmission issuppressed.
 12. The method of claim 1, further comprising determining toenter the reduced activity mode for a period of time based on acorrespondence between an identifier for the terminal device and anidentity associated with the at least one reference symbol for whichtransmission is suppressed.
 13. The method of claim 12, wherein theidentity associated with the at least one reference symbol for whichtransmission is suppressed uniquely identifies the terminal device. 14.The method of claim 12, wherein the identity associated with the atleast one reference symbol for which transmission is suppressedidentifies a group of terminal devices of which the terminal device is amember.
 15. The method of claim 14, wherein an association between theterminal device and the group of terminal devices of which the terminaldevice is a member is established by signalling between the base stationand the terminal device.
 16. The method of claim 14, wherein anassociation between the terminal device and the group of terminaldevices of which the terminal device is a member is pre-defined for thewireless telecommunications system.
 17. The method of claim 1, whereinthe at least one reference symbol is identified as being suppressedbased on it not being received or being received with less power thanother reference symbols.
 18. The method of claim 1, wherein the radiointerface has a downlink radio frame structure comprising radiosubframes.
 19. The method of claim 18, wherein the period of timecorresponds with a number of subframes starting at an offset definedrelative to a subframe in which a reference symbol is suppressed. 20.The method of claim 18, wherein the at least one reference symbol forwhich transmission is suppressed comprises at least one reference symbolin each one of more than one subframe.
 21. The method of claim 1,further comprising deriving further information communicated from thebase station to the terminal device based on the at least one referencesymbol for which transmission is suppressed.
 22. The method of claim 1,further comprising the terminal device transmitting signaling to thebase station during the reduced activity mode to request resources forsubsequent communications between the base station and the terminaldevice.
 23. The method of claim 1, further comprising the terminaldevice transmitting a channel quality indicator, CQI, to the basestation on exit from the reduced activity mode.
 24. The method of claim1, wherein the reduced activity mode is a mode in which the terminaldevice is configured to decode fewer transmissions from the base stationthan when the terminal device is not in the reduced activity mode. 25.The method of claim 1, wherein the terminal device continues to decodeat least one of synchronisation information, system information, orreference symbols when in the reduced activity mode.
 26. A terminaldevice for use in a wireless telecommunications system employing a radiointerface including an arrangement of downlink reference symbols, theterminal device comprising: a communication interface configured toreceive reference symbols transmitted by a base station; and circuitryconfigured to identify that transmission by the base station of at leastone reference symbol from the arrangement of downlink reference symbolsis suppressed; determine a period of time for which to enter a reducedactivity mode based on the identified at least one reference symbol forwhich transmission is suppressed; and initiate the reduced activity modefor the determined period of time, wherein the radio interface comprisesa plurality of Orthogonal Frequency Division Multiplexing (OFDM)sub-carriers spanning a system frequency bandwidth, the radio interfacesupports a first carrier for communicating with a first class ofterminal device using a first group of the OFDM sub-carriers distributedacross the system frequency bandwidth, and a second carrier forcommunicating with a second class of terminal device on a second groupof the OFDM sub-carriers distributed across a restricted frequencybandwidth that is narrower than and within the system frequencybandwidth, and the terminal device is a terminal device of the secondclass operating on the second carrier.
 27. The terminal device of claim26, configured such that the determined period of time for which toenter the reduced activity mode is based on an association betweendifferent ones of the reference symbols and different potential periodsof time for entering the reduced activity mode.
 28. The terminal deviceof claim 26, wherein the association between different ones of thereference symbols and different potential periods of time for enteringthe reduced activity mode is pre-defined for the wirelesstelecommunications system.
 29. The terminal device of claim 26, whereinthe association between different ones of the reference symbols anddifferent potential periods of time for entering the reduced activitymode is communicated to the terminal device from the base station.
 30. Aterminal device for use in a wireless telecommunications systememploying a radio interface including an arrangement of downlinkreference symbols, the terminal device comprising: a communicationinterface configured to receive reference symbols transmitted by a basestation, and circuitry configured to identify that transmission by thebase station of at least one reference symbol from the arrangement ofdownlink reference symbols is suppressed; determine a period of time forwhich to enter a reduced activity mode based on the identified at leastone reference symbol for which transmission is suppressed; and initiatethe reduced activity mode for the determined period of time, wherein thedetermined period of time for which to enter the reduced activity modeis based on an association between different ones of the referencesymbols and different potential periods of time for entering the reducedactivity mode, and the association between different ones of thereference symbols and different potential periods of time for enteringthe reduced activity mode is defined in a look-up table.
 31. A terminaldevice for use in a wireless telecommunications system employing a radiointerface including an arrangement of downlink reference symbols, theterminal device comprising: a communication interface configured toreceive reference symbols transmitted by a base station; and circuitryconfigured to identify that transmission by the base station of at leastone reference symbol from the arrangement of downlink reference symbolsis suppressed; determine a period of time for which to enter a reducedactivity mode based on the identified at least one reference symbol forwhich transmission is suppressed; and initiate the reduced activity modefor the determined period of time, wherein the at least one referencesymbol for which transmission is suppressed comprises more than onereference symbol and wherein the terminal device is configured such thatthe period of time for entering the reduced activity mode is determinedaccording to a mapping between different combinations of referencesymbols and a plurality of potential periods of time for entering thereduced activity mode.
 32. A terminal device for use in a wirelesstelecommunications system employing a radio interface including anarrangement of downlink reference symbols, the terminal devicecomprising: a communication interface configured to receive referencesymbols transmitted by a base station; and circuitry configured toidentify that transmission by the base station of at least one referencesymbol from the arrangement of downlink reference symbols is suppressed;determine a period of time for which to enter a reduced activity modebased on the identified at least one reference symbol for whichtransmission is suppressed; and initiate the reduced activity mode forthe determined period of time, wherein the communication interface isconfigured to receive the reference symbols on transmissions frommultiple antenna ports of the base station, and the circuitry isconfigured to determine the period of time for entering the reducedactivity mode based on which antenna port is associated with the atleast one reference symbol for which transmission is suppressed.
 33. Theterminal device of claim 26, wherein the reference symbols comprise atleast one of cell-specific reference symbols, terminal device specificreference symbols, demodulation reference symbols, channel stateinformation reference symbols, or positioning reference symbols.
 34. Theterminal device of claim 26, configured such that a start time for aperiod of time for entering the reduced activity mode relative to a timeat which the transmission of the at least one reference symbol issuppressed is also determined based on the at least one reference symbolfor which transmission is suppressed.
 35. The terminal device of claim26, further configured to determine at least one further period of timefor which to enter a reduced activity mode based on an identified atleast one reference symbol for which transmission is suppressed.
 36. Theterminal device of claim 35, wherein the determined period of time andat least one further period of time follow a pattern defined accordingto the at least one reference symbol for which transmission issuppressed.
 37. The terminal device of claim 26, further configured todetermine to enter the reduced activity mode for a period of time basedon a correspondence between an identifier for the terminal device and anidentity associated with the at least one reference symbol for whichtransmission is suppressed.
 38. The terminal device of claim 37, whereinthe identity associated with the at least one reference symbol for whichtransmission is suppressed uniquely identifies the terminal device. 39.The terminal device of claim 37, wherein the identity associated withthe at least one reference symbol for which transmission is suppressedidentifies a group of terminal devices of which the terminal device is amember.
 40. The terminal device of claim 39, configured such that anassociation between the terminal device and the group of terminaldevices of which the terminal device is a member is established bysignalling between the base station and the terminal device.
 41. Theterminal device of claim 39, wherein an association between the terminaldevice and the group of terminal devices of which the terminal device isa member is pre-defined for the wireless telecommunications system. 42.The terminal device of claim 26, configured such that the at least onereference symbol is identified as being suppressed based on it not beingreceived or being received with less power than other reference symbols.43. The terminal device of claim 26, wherein the radio interface has adownlink radio frame structure comprising radio subframes.
 44. Theterminal device of claim 43, wherein the period of time corresponds witha number of subframes starting at an offset defined relative to asubframe in which a reference symbol is suppressed.
 45. The terminaldevice of claim 43, wherein the at least one reference symbol for whichtransmission is suppressed comprises at least one reference symbol ineach one of more than one subframe.